Recommit r358211 "[X86] Use FILD/FIST to implement i64 atomic load on 32-bit targets...

Recommit r358211 "[X86] Use FILD/FIST to implement i64 atomic load on 32-bit targets with X87, but no SSE2"

With correct test checks this time.

If we have X87, but not SSE2 we can atomicaly load an i64 value into the significand of an 80-bit extended precision x87 register using fild. We can then use a fist instruction to convert it back to an i64 integ

This matches what gcc and icc do for this case and removes an existing FIXME.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358214 91177308-0d34-0410-b5e6-96231b3b80d8