[InstCombine] shrink switch conditions better (PR24766)

https://llvm.org/bugs/show_bug.cgi?id=24766#c2

This removes a hack that was added for the benefit of x86 codegen. 
It prevented shrinking the switch condition even to smaller legal (DataLayout) types.
We have a safety mechanism in CGP after:
http://reviews.llvm.org/rL251857
...so we're free to use the optimal (smallest) IR type now.

Differential Revision: http://reviews.llvm.org/D12965

llvm-svn: 274233