[NVPTX] Enhance vectorization of ld.param & st.param

[NVPTX] Enhance vectorization of ld.param & st.param

Since function parameters and return values are passed via param space, we
can force special alignment for values hold in it which will add vectorization
options. This change may be done if the function has private or internal
linkage. Special alignment is forced during 2 phases.

1) Instruction selection lowering. Here we use special alignment for function
   prototypes (changing both own return value and parameters alignment), call
   lowering (changing both callee's return value and parameters alignment).

2) IR pass nvptx-lower-args. Here we change alignment of byval parameters that
   belong to param space (or are casted to it). We only handle cases when all
   uses of such parameters are loads from it. For such loads, we can change the
   alignment according to special type alignment and the load offset. Then,
   load-store-vectorizer IR pass will perform vectorization where alignment
   allows it.

Special alignment calculated as maximum from default ABI type alignment and
alignment 16. Alignment 16 is chosen because it's the maximum size of
vectorized ld.param & st.param.

Before specifying such special alignment, we should check if it is a multiple
of the alignment that the type already has. For example, if a value has an
enforced alignment of 64, default ABI alignment of 4 and special alignment
of 16, we should preserve 64.

This patch will be followed by a refactoring patch that removes duplicating
code in handling byval and non-byval arguments.

Differential Revision: https://reviews.llvm.org/D120129