[AMDGPU] Simplify AMDGPUAnnotateUniformValues::visitLoadInst

Always set uniform metadata on the pointer if it is an instruction, but
otherwise do not bother to create a trivial getelementptr instruction,
because AMDGPUInstrInfo::isUniformMMO can already detect that various
non-instruction pointers are uniform.

Most of the test case churn is from tests that used undef as a pointer,
which AMDGPUInstrInfo::isUniformMMO treats as uniform.

Differential Revision: https://reviews.llvm.org/D118909