[AMDGPU] Add support for multi-dword s.buffer.load intrinsic

Summary:
Patch by Marek Olsak and David Stuttard, both of AMD.

This adds a new amdgcn intrinsic supporting s.buffer.load, in particular
multiple dword variants. These are convenient to use from some front-end
implementations.

Also modified the existing llvm.SI.load.const intrinsic to common up the
underlying implementation.

This modification also requires that we can lower to non-uniform loads correctly
by splitting larger dword variants into sizes supported by the non-uniform
versions of the load.

V2: Addressed minor review comments.
V3: i1 glc is now i32 cachepolicy for consistency with buffer and
    tbuffer intrinsics, plus fixed formatting issue.
V4: Added glc test.

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D51098

Change-Id: I83a6e00681158bb243591a94a51c7baa445f169b
llvm-svn: 340684