[PATCH, i386]: Fix PR target/34682, 70% slowdown with SSE enabled

[Jan Hubicka]

> Hello!
> 
> The testcase in the PR exposed a problem, where xorb with partial QImode 
> memory access was used to flip the sign bit of DFmode value. This 
> partial memory access resulted in ~70% slowdown when -mfpmath=sse was 
> used. The same problem affected 64bit binary, although ~20% slowdown was 
> measured on a Core2 processor.

Why do you see slowdown on SFmode operations and 64bit DFmode where
matching size integer operation can be used?

Honza