[AMDGPU] Handle natively unsupported types in addrspace(7) lowering

The current lowering for ptr addrspace(7) assumed that the instruction
selector can handle arbtrary LLVM types, which is not the case. Code
generation can't deal with
- Values that aren't 8, 16, 32, 64, 96, or 128 bits long
- Aggregates (this commit only handles arrays of scalars, more may come)
- Vectors of more than one byte
- 3-word values that aren't a vector of 3 32-bit values (for axample, a
  <6 x half>)

This commit adds a buffer contents type legalizer that adds the needed
bitcasts, zero-extensions, and splits into subcompnents needed to convert a
load or store operation into one that can be successfully lowered through
code generation.

In the long run, some of the involved bitcasts (though potentially not
the buffer operation splitting) ought to be handled by the instruction
legalizer, but SelectionDAG makes this difficult.

It also takes advantage of the new `nuw` flag on `getelementptr` when
lowering GEPs to offset additions.

We don't currently plumb through `nsw` on GEPs since that should likely
be a separate change and would require declaring what we mean by
"the address" in the context of the GEP guarantees.

Author: krzysz00