[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356464
91177308-0d34-0410-b5e6-
96231b3b80d8
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