Upgrade the ugly darwin 64-bit bswap idiom (bswap %eax / bswap %edx /

xchgl %eax, %edx) to llvm.bswap.i64.  This compiles:

long long test2(long long A) {
  return _OSSwapInt64(A);
}

to:

_test2:
        movl 8(%esp), %eax
        movl 4(%esp), %edx
        bswapl %eax
        bswapl %edx
        ret

instead of:

_test2:
        movl 8(%esp), %edx
        movl 4(%esp), %eax
        bswap   %eax
        bswap   %edx
        xchgl   %eax, %edx
        ret

GCC manages (with -fomit-frame-pointer) the uglier:

_test2:
        subl    $4, %esp
        movl    8(%esp), %eax
        movl    12(%esp), %edx
        bswap   %eax
        bswap   %edx
        xchgl   %eax, %edx
        addl    $4, %esp
        ret

llvm-svn: 32001