[InstCombine] move/add tests for fmul distribution; NFC

There are at least 3 problems:
1. We're distributing across large patterns, but fail to do that for the minimal patterns.
2. We're not checking uses, so we may create more instructions than we eliminate.
3. We should be able to do these transforms with less than full 'fast' fmuls.

llvm-svn: 328152