[Constants] add identity constants for fadd/fmul

As the test diffs show, the current users of getBinOpIdentity()
are InstCombine and Reassociate. SLP vectorizer is a candidate
for using this functionality too (D28907).

The InstCombine shuffle improvements are part of the planned
enhancements noted in D48830.

InstCombine actually has several other uses of getBinOpIdentity() 
via SimplifyUsingDistributiveLaws(), but we don't call that for 
any FP ops. Fixing that might be another part of removing the
custom reassociation in InstCombine that is only done for fadd+fmul.

llvm-svn: 336215