[InstCombine] allow fmul fold with less than 'fast'

This is a retry of r326502 with updates to the reassociate 
test file that I missed the first time.

@test15_reassoc in the supposed -reassociate test file 
(except that it tests 2 other passes too...) shows that
there's no clear responsiblity for reassociation transforms.

Instcombine now gets that case, but only because the
constant values are identical. Otherwise, it would still
miss that pattern. 

Reassociate doesn't get that case because it hasn't been 
updated to use less than 'fast' FMF.

llvm-svn: 326513