[InstCombine] don't change the size of a select if it would mismatch its condition operands' sizes

Don't always:
cast (select (cmp x, y), z, C) --> select (cmp x, y), (cast z), C'

This is something that came up as far back as D26556, and I lost track of it. 
I suspect that this transform is part of the underlying problem that is 
inspiring some of the recent proposals that seek to match larger patterns 
that include a cast op. Even if that's not true, this transform causes
problems for codegen (particularly with vector types).

A transform to actively match the size of cmp and select operand sizes should
follow. This patch just removes the harmful canonicalization in the other
direction.

Differential Revision: https://reviews.llvm.org/D47163

llvm-svn: 333611