[InstSimplify] reorder methods; NFC

I'm trying to refactor some shared code for integer div/rem,
but I keep having to scroll through fdiv. The FP ops have
nothing in common with the integer ops, so I'm moving FP
below everything else. 

While here, improve a couple of comments and fix some formatting.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312913 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed