[AMDGPU] Ban i8 min3 promotion.

I found this really weird WWM-related case whereby through the WWM
transformations our isel lowering was trying to promote 2 min's into a
min3 for the i8 type, which our hardware doesn't support.

The new min3_i8.ll test case would previously spew the error:

PromoteIntegerResult #0: t69: i8 = SMIN3 t70, Constant:i8<0>, t68

Before the simple fix to our isel lowering to not do it for i8 MVT's.

Differential Revision: https://reviews.llvm.org/D59543

llvm-svn: 356464