[amdgpu] Update med3 combine to skip i64

[amdgpu] Update med3 combine to skip i64

Fixes an assumption that a type which is not i32 will be i16. This asserts
when trying to sign/zero extend an i64 to i32.

Test case was cut down from an openmp application. Variations on it are hit by
other combines before reaching the problematic one, e.g. replacing the
immediate values with other function arguments changes the codegen path and
misses this combine.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D98872