[InstCombine] Add checking of i1 types when converting select instructions into zext/sext instructions

[congzhe]

Summary

When doing select-to-zext/sext transformations, we should not handle
TrueVal and FalseVal of i1 type otherwise it would result in zext/sext i1 to i1.

Note that the current trunk code does not break since the checking currently does exist
but is implicit. When we have TrueVal and FalseVal of i1 type, this case will be captured
in the previous code block:
https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp#L2629

Therefore no unit test is provided in this patch. For maintenance purposes however,
this patch makes the checking explicit instead of implicit.

[spatel]

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

2612

Extra && ?

[spatel]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

5

Nit: put a period at the end of sentence.

[congzhe]

In D93272#2466125, @spatel wrote:

LGTM

Thanks for the review! Committed the patch.

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

 //===- InstCombineSelect.cpp ----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the visitSelect function.
 //
 //===----------------------------------------------------------------------===//
 
 #include "InstCombineInternal.h"
Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   }
 
   // Selecting between two integer or vector splat integer constants?
   //
   // Note that we don't handle a scalar select of vectors:
   // select i1 %c, <2 x i8> <1, 1>, <2 x i8> <0, 0>
   // because that may need 3 instructions to splat the condition value:
   // extend, insertelement, shufflevector.
-  if (SelType->isIntOrIntVectorTy() &&
+  //
+  // Do not handle i1 TrueVal and FalseVal otherwise would result in
+  // zext/sext i1 to i1.
+  if (SelType->isIntOrIntVectorTy() && !SelType->isIntOrIntVectorTy(1) &&
       CondVal->getType()->isVectorTy() == SelType->isVectorTy()) {
     // select C, 1, 0 -> zext C to int
     if (match(TrueVal, m_One()) && match(FalseVal, m_Zero()))
       return new ZExtInst(CondVal, SelType);
 
     // select C, -1, 0 -> sext C to int
     if (match(TrueVal, m_AllOnes()) && match(FalseVal, m_Zero()))
       return new SExtInst(CondVal, SelType);