[InstCombine] refactor optimizeIntToFloatBitCast() ; NFCI

The logic for handling the pattern without a shift is identical
to the logic for handling the pattern with a shift if you set 
the shift amount to zero for the former.

This should make it easier to see that we probably don't even need
optimizeIntToFloatBitCast(). 

If we call something like foldVecTruncToExtElt() from visitTrunc(),
we'll solve PR25543:
https://llvm.org/bugs/show_bug.cgi?id=25543

llvm-svn: 253403