[X86] Add test for PR49587 (NFC)

Shows a miscompile with FastISel.

(cherry picked from commit 0d814ca)

Author: nikic

llvm/test/CodeGen/X86/pr49587.ll

@@ -0,0 +1,32 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -O0 -fast-isel -mtriple=x86_64-- < %s | FileCheck %s
+
+define i32 @test(i64 %arg) nounwind {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    subq $1, %rdi
+; CHECK-NEXT:    setb %al
+; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    movl %eax, {{[-0-9]+}}(%r{{[sb]}}p) # 4-byte Spill
+; CHECK-NEXT:    jb .LBB0_2
+; CHECK-NEXT:  # %bb.1: # %no_overflow
+; CHECK-NEXT:    movl $1, %eax
+; CHECK-NEXT:    movl %eax, {{[-0-9]+}}(%r{{[sb]}}p) # 4-byte Spill
+; CHECK-NEXT:    jmp .LBB0_2
+; CHECK-NEXT:  .LBB0_2: # %merge
+; CHECK-NEXT:    movl {{[-0-9]+}}(%r{{[sb]}}p), %eax # 4-byte Reload
+; CHECK-NEXT:    retq
+entry:
+  %usubo = tail call { i64, i1 } @llvm.usub.with.overflow.i64(i64 %arg, i64 1)
+  %overflow = extractvalue { i64, i1 } %usubo, 1
+  br i1 %overflow, label %merge, label %no_overflow
+
+no_overflow:
+  br label %merge
+
+merge:
+  %phi = phi i32 [ 1, %no_overflow ], [ 0, %entry ]
+  ret i32 %phi
+}
+
+declare { i64, i1 } @llvm.usub.with.overflow.i64(i64, i64)