Preserve param alignment in NVPTXLowerArgs pass.
NVPTXLowerArgs works as follows.
- Create a regular alloca with alignment identical to arg.
- Copy arg from param space (and ASC'ing it from generic AS first) to the alloca (it's still in generic AS).
- Replace loads of arg with loads of alloca.
The bug here is that we did not preserve the arg's alignment when
loading from the alloca.
The impact of this bug is that sometimes param loads would be lowered as
a series of u8 loads, because we're incorrectly assuming everything has