[ARM] Consider undefined-on-NaN conditions in checkVSELConstraints

This function was not checking for the condition code variants which are
undefined if either input is NaN, so we were missing selection of the VSEL
instruction in some cases when using -fno-honor-nans or -ffast-math.

Differential revision: https://reviews.llvm.org/D58812

llvm-svn: 355199