[SPIRV] Prevent creation of jump tables from switch (#82287)

This PR is to prevent creation of jump tables from switch. The reason is
that SPIR-V doesn't know how to lower jump tables, and a sequence of
commands that IRTranslator generates for switch via jump tables breaks
SPIR-V Backend code generation with complains to G_BRJT. The next
example is the shortest code to break SPIR-V Backend code generation in
this way:

```
target datalayout = "e-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-n8:16:32:64"
target triple = "spir64-unknown-unknown"

define spir_func void @foo(i32 noundef %val) {
entry:
  switch i32 %val, label %sw.epilog [
    i32 0, label %sw.bb
    i32 1, label %sw.bb2
    i32 2, label %sw.bb3
    i32 3, label %sw.bb4
  ]
sw.bb:
  br label %sw.epilog
sw.bb2:
  br label %sw.epilog
sw.bb3:
  br label %sw.epilog
sw.bb4:
  br label %sw.epilog
sw.epilog:
  ret void
}
```

To resolve the issue we set a high lower limit for number of blocks in a
jump table via getMinimumJumpTableEntries() and prevent undesirable (or
rather unsupported at the moment) path of code generation.

Author: VyacheslavLevytskyy

llvm/lib/Target/SPIRV/SPIRVISelLowering.h

@@ -31,6 +31,9 @@ class SPIRVTargetLowering : public TargetLowering {
     return true;
   }
 
+  // prevent creation of jump tables
+  bool areJTsAllowed(const Function *) const override { return false; }
+
   // This is to prevent sexts of non-i64 vector indices which are generated
   // within general IRTranslator hence type generation for it is omitted.
   MVT getVectorIdxTy(const DataLayout &DL) const override {

llvm/test/CodeGen/SPIRV/switch-no-jump-table.ll

@@ -0,0 +1,30 @@
+; The test is to check that jump tables are not generated from switch
+
+; RUN: llc -O0 -mtriple=spirv32-unknown-unknown %s -o - | FileCheck %s
+; RUN: %if spirv-tools %{ llc -O0 -mtriple=spirv64-unknown-unknown %s -o - -filetype=obj | spirv-val %}
+
+; CHECK: OpSwitch %[[#]] %[[Label:]]
+; CHECK-4: OpBranch %[[Label]]
+
+target datalayout = "e-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-n8:16:32:64"
+target triple = "spir64-unknown-unknown"
+
+define spir_func void @foo(i32 noundef %val) {
+entry:
+  switch i32 %val, label %sw.epilog [
+    i32 0, label %sw.bb
+    i32 1, label %sw.bb2
+    i32 2, label %sw.bb3
+    i32 3, label %sw.bb4
+  ]
+sw.bb:
+  br label %sw.epilog
+sw.bb2:
+  br label %sw.epilog
+sw.bb3:
+  br label %sw.epilog
+sw.bb4:
+  br label %sw.epilog
+sw.epilog:
+  ret void
+}