Rename "fpaccuracy" metadata to the more generic "fpmath". That's because I'm

thinking of generalizing it to be able to specify other freedoms beyond accuracy
(such as that NaN's don't have to be respected).  I'd like the 3.1 release (the
first one with this metadata) to have the more generic name already rather than
having to auto-upgrade it in 3.2.

llvm-svn: 154745