[AArch64] Fix Copy Elemination for negative values

Redundant Copy Elimination was eliminating a MOVi32imm -1 when it
determined that the value of the destination register is already -1.
However, it didn't take into account that the MOVi32imm zeroes the upper
32 bits (which are FFFFFFFF) and therefore cannot be eliminated.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D93100

Author: tmatheson-arm

llvm/lib/Target/AArch64/AArch64RedundantCopyElimination.cpp

@@ -408,6 +408,11 @@ bool AArch64RedundantCopyElimination::optimizeBlock(MachineBasicBlock *MBB) {
                          O.getReg() != CmpReg;
                 }))
               continue;
+
+            // Don't remove a move immediate that implicitly defines the upper
+            // bits as different.
+            if (TRI->isSuperRegister(DefReg, KnownReg.Reg) && KnownReg.Imm < 0)
+              continue;
           }
 
           if (IsCopy)

llvm/test/CodeGen/AArch64/machine-copy-remove.mir

@@ -536,13 +536,13 @@ body:             |
   bb.2:
     RET_ReallyLR
 ...
-# Eliminate redundant MOVi32imm -1 in bb.1
+# Don't eliminate redundant MOVi32imm -1 in bb.1: the upper bits are nonzero.
 # Note: 64-bit compare/32-bit move imm
 # Kill marker should be removed from compare.
 # CHECK-LABEL: name: test21
-# CHECK: ADDSXri $x0, 1, 0, implicit-def $nzcv
+# CHECK: ADDSXri killed $x0, 1, 0, implicit-def $nzcv
 # CHECK: bb.1:
-# CHECK-NOT: MOVi32imm
+# CHECK: MOVi32imm
 name:            test21
 tracksRegLiveness: true
 body:             |