[IfConversion] Pre-commit testcase for !HasFallThrough bug. NFC

Adding a test case showing that we can't assume that
!HasFallThrough implies that there is no fallthrough exit
in case analyzeBranch returned true (true == "could not analyze").

Author: bjope

llvm/test/CodeGen/ARM/ifcvt_unanalyzable_fallthrough.mir

@@ -0,0 +1,114 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=thumbv7-apple-ios -run-pass=if-converter %s -o - | FileCheck %s
+
+# Testcase with unanalyzable branches (that may fallthrough) in the BB
+# following the diamond/triangle.
+
+# Goal here is to showcase a problem seen in the IfConverter when
+# AnalyzeBranch is indicating that the branches couldn't be analyzed. Problem
+# was originally seen for an out-of-tree target, and here we use ARM and a
+# MBB with two conditional jumps to make AnalyzeBranch return false.
+#
+# The problem was that if-converter when analyzing branches was using a
+# variable named HasFallThrough, to remember that an MBB could fallthrough to
+# the textual successor. When HasFallThrough is set we know that there are
+# fallthrough exits, but the opposite is not guaranteed. If
+# HasFallThrough==false there could still be fallthrough exists in situations
+# when analyzeBranch found unanalyzable branches. There were however a couple
+# of places in the code that checked !HasFallThrough assuming that it would
+# imply that there was no fallthrough exit.
+#
+# As a consequence we could end up merging blocks at the end of a converted
+# diamond/triangle and while doing that we messed up when fixing up the CFG
+# related to fallthrough edges. For the test cases below we incorrectly ended
+# up ended up with a fallthrough from the MBBs with two Bcc instructions to
+# the MBB with the STRH after if conversion.
+#
+---
+name:            avoidMergeBlockDiamond
+body:             |
+  ; CHECK-LABEL: name: avoidMergeBlockDiamond
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.2(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   $sp = tADDspi $sp, 2, 1 /* CC::ne */, $cpsr
+  ; CHECK-NEXT:   $sp = tADDspi $sp, 1, 0 /* CC::eq */, $cpsr, implicit $sp
+  ; CHECK-NEXT:   $sp = tADDspi $sp, 3, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT:   tBcc %bb.2, 1 /* CC::ne */, $cpsr
+  ; CHECK-NEXT:   tBcc %bb.2, 1 /* CC::ne */, $cpsr
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   STRH $sp, $sp, $noreg, 0, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT:   tB %bb.1, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   tBX_RET 14 /* CC::al */, $noreg
+  bb.0:
+    tBcc %bb.2, 1, $cpsr
+
+  bb.1:
+    $sp = tADDspi $sp, 1, 14, _
+    tB %bb.4, 14, $noreg
+
+  bb.2:
+    $sp = tADDspi $sp, 2, 14, _
+    tB %bb.4, 14, $noreg
+
+  bb.3:
+    STRH $sp, $sp, $noreg, 0, 14, $noreg
+    tB %bb.3, 14, $noreg
+
+  bb.4:
+    $sp = tADDspi $sp, 3, 14, _
+    tBcc %bb.5, 1, $cpsr
+    tBcc %bb.5, 1, $cpsr
+
+  bb.5:
+  successors:
+    tBX_RET 14, _
+...
+
+# Similar to the above, but with a triangle.
+---
+name:            avoidMergeBlockTriangle
+body:             |
+  ; CHECK-LABEL: name: avoidMergeBlockTriangle
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.2(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   $sp = tADDspi $sp, 1, 1 /* CC::ne */, $cpsr
+  ; CHECK-NEXT:   $sp = tADDspi $sp, 2, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT:   tBcc %bb.2, 1 /* CC::ne */, $cpsr
+  ; CHECK-NEXT:   tBcc %bb.2, 1 /* CC::ne */, $cpsr
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   STRH $sp, $sp, $noreg, 0, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT:   tB %bb.1, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   tBX_RET 14 /* CC::al */, $noreg
+  bb.0:
+    tBcc %bb.1, 1, $cpsr
+    tB %bb.3, 14, $noreg
+
+  bb.1:
+    $sp = tADDspi $sp, 1, 14, _
+    tB %bb.3, 14, $noreg
+
+  bb.2:
+    STRH $sp, $sp, $noreg, 0, 14, $noreg
+    tB %bb.2, 14, $noreg
+
+  bb.3:
+    $sp = tADDspi $sp, 2, 14, _
+    tBcc %bb.4, 1, $cpsr
+    tBcc %bb.4, 1, $cpsr
+
+  bb.4:
+  successors:
+    tBX_RET 14, _
+...