Implement a special algorithm for converting uint_to_fp for i32 values on

X86. This code:

void f() {
  uint32_t x;
  float y = (float)x;
}

used to be:

     movl     %eax, -8(%ebp)
     movl     [2^52 double], -4(%ebp)
     movsd    -8(%ebp), %xmm0
     subsd    [2^52 double], %xmm0
     cvtsd2ss %xmm0, %xmm0

Is now:

   movsd        [2^52 double], %xmm0
   movsd        %xmm0, %xmm1
   movd         %ecx, %xmm2
   orps         %xmm2, %xmm1
   subsd        %xmm0, %xmm1
   cvtsd2ss     %xmm1, %xmm0

This is faster on X86. Note that there's an extra load of %xmm0 into %xmm1. That
will be fixed in a later coalescer fix.

llvm-svn: 62404