[InstCombine] Fix crashing from (icmp (bitcast ([su]itofp X)), Y)

This fixes a class of bugs introduced by D44367, 
which transforms various cases of icmp (bitcast ([su]itofp X)), Y to icmp X, Y. 
If the bitcast is between vector types with a different number of elements, 
the current code will produce bad IR along the lines of: icmp <N x i32> ..., <M x i32> <...>.

This patch suppresses the transform if the bitcast changes the number of vector elements.

Patch by: @AndrewScheidecker (Andrew Scheidecker)

Differential Revision: https://reviews.llvm.org/D57871

llvm-svn: 353467