[X86] Add signed aliases for popcnt intrinsics

[mkuper]

Summary

The Intel manual documents both an unsigned form (_mm_popcnt_u32) and a signed form (_popcnt32) of the intrinsic.
Add the missing signed form.

[mkuper]

And now, with a test that actually checks what it ought to check.

[delena]

LGTM

lib/Headers/popcntintrin.h

 #define __DEFAULT_FN_ATTRS __attribute__((__always_inline__, __nodebug__, __target__("popcnt")))
 
 static __inline__ int __DEFAULT_FN_ATTRS
 _mm_popcnt_u32(unsigned int __A)
 {
   return __builtin_popcount(__A);
 }
 
+static __inline__ int __DEFAULT_FN_ATTRS
+_popcnt32(int __A)
+{
+  return __builtin_popcount(__A);
+}
+
 #ifdef __x86_64__
 static __inline__ long long __DEFAULT_FN_ATTRS
 _mm_popcnt_u64(unsigned long long __A)
 {
   return __builtin_popcountll(__A);
 }
+
+static __inline__ long long __DEFAULT_FN_ATTRS
+_popcnt64(long long __A)
+{
+  return __builtin_popcountll(__A);
+}
 #endif /* __x86_64__ */
 
 #undef __DEFAULT_FN_ATTRS
 
 #endif /* _POPCNTINTRIN_H */

test/CodeGen/popcnt-builtins.c

 // RUN: %clang_cc1 %s -triple=x86_64-apple-darwin -target-feature +popcnt -emit-llvm -o - | FileCheck %s
 
 // Don't include mm_malloc.h, it's system specific.
 #define __MM_MALLOC_H
 
 #include <x86intrin.h>
 
 unsigned int test_mm_popcnt_u32(unsigned int __X) {
-  // CHECK: @llvm.ctpop.i32
+  //CHECK: call i32 @llvm.ctpop.i32
   return _mm_popcnt_u32(__X);
 }
 
+unsigned int test_popcnt_32(int __X) {
+  //CHECK: call i32 @llvm.ctpop.i32
+  return _popcnt32(__X);
+}
+
 unsigned long long test_mm_popcnt_u64(unsigned long long __X) {
-  // CHECK: @llvm.ctpop.i64
+  //CHECK: call i64 @llvm.ctpop.i64
   return _mm_popcnt_u64(__X);
 }
+
+unsigned long long test_popcnt_64(long long __X) {
+  //CHECK: call i64 @llvm.ctpop.i64
+  return _popcnt64(__X);
+}