[X86] Enable isel to use the PAUSE instruction even when SSE2 is disabled. Clang part

[craig.topper]

Summary

On older processors this instruction encoding is treated as a NOP.

MSVC doesn't disable intrinsics based on features the way clang/gcc does. Because the PAUSE instruction encoding doesn't crash older processors, some software out there uses these intrinsics without checking for SSE2.

This change also seems to also be consistent with gcc behavior.

Fixes PR34079

[RKSimon]

Codegen test case on no-sse2 target?

[craig.topper]

Add test case

[RKSimon]

LGTM

include/clang/Basic/BuiltinsX86.def

 TARGET_BUILTIN(__builtin_ia32_cvtps2dq, "V4iV4f", "", "sse2")
 TARGET_BUILTIN(__builtin_ia32_cvttps2dq, "V4iV4f", "", "sse2")
 TARGET_BUILTIN(__builtin_ia32_clflush, "vvC*", "", "sse2")
 TARGET_HEADER_BUILTIN(_mm_clflush, "vvC*", "h", "emmintrin.h", ALL_LANGUAGES, "sse2")
 TARGET_BUILTIN(__builtin_ia32_lfence, "v", "", "sse2")
 TARGET_HEADER_BUILTIN(_mm_lfence, "v", "h", "emmintrin.h", ALL_LANGUAGES, "sse2")
 TARGET_BUILTIN(__builtin_ia32_mfence, "v", "", "sse2")
 TARGET_HEADER_BUILTIN(_mm_mfence, "v", "h", "emmintrin.h", ALL_LANGUAGES, "sse2")
-TARGET_BUILTIN(__builtin_ia32_pause, "v", "", "sse2")
-TARGET_HEADER_BUILTIN(_mm_pause, "v", "h", "emmintrin.h", ALL_LANGUAGES, "sse2")
+TARGET_BUILTIN(__builtin_ia32_pause, "v", "", "")
+TARGET_HEADER_BUILTIN(_mm_pause, "v", "h", "emmintrin.h", ALL_LANGUAGES, "")
 TARGET_BUILTIN(__builtin_ia32_pmuludq128, "V2LLiV4iV4i", "", "sse2")
 TARGET_BUILTIN(__builtin_ia32_psraw128, "V8sV8sV8s", "", "sse2")
 TARGET_BUILTIN(__builtin_ia32_psrad128, "V4iV4iV4i", "", "sse2")
 TARGET_BUILTIN(__builtin_ia32_psrlw128, "V8sV8sV8s", "", "sse2")
 TARGET_BUILTIN(__builtin_ia32_psrld128, "V4iV4iV4i", "", "sse2")
 TARGET_BUILTIN(__builtin_ia32_psrlq128, "V2LLiV2LLiV2LLi", "", "sse2")
 TARGET_BUILTIN(__builtin_ia32_psllw128, "V8sV8sV8s", "", "sse2")
 TARGET_BUILTIN(__builtin_ia32_pslld128, "V4iV4iV4i", "", "sse2")

test/CodeGen/pause.c

+// RUN: %clang_cc1 -ffreestanding %s -triple=i386-pc-win32 -target-feature -sse2 -emit-llvm -o - -Wall -Werror | FileCheck %s
+// RUN: %clang_cc1 -ffreestanding %s -triple=i386-pc-win32 -target-feature +sse2 -emit-llvm -o - -Wall -Werror | FileCheck %s
+
+
+#include <x86intrin.h>
+
+void test_mm_pause() {
+  // CHECK-LABEL: test_mm_pause
+  // CHECK: call void @llvm.x86.sse2.pause()
+  return _mm_pause();
+}