AMDGPU: Add Assert[SZ]Ext during argument load creation

AMDGPU: Add Assert[SZ]Ext during argument load creation

For i16 zeroext arguments when i16 was a legal type, the
known bits information from the truncate was lost. Insert
a zeroext so the known bits optimizations work with the 32-bit
loads.

Fixes code quality regressions vs. SI in min.ll test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291461 91177308-0d34-0410-b5e6-96231b3b80d8