[PATCH] Use rcpss/rcpps (X86) to speed up reciprocal calcs (PR21385)

[hfinkel at anl.gov]

> This is very similar to the rsqrt optimization enabled in D5658 ( http://reviews.llvm.org/rL220570 ).

Yes, indeed it seems that way (when you commit, make sure you mention the commit revision corresponding to D5658 in the commit message).

LGTM.

http://reviews.llvm.org/D6175