[AMDGPU] Allow struct.buffer.*.format intrinsics to accept i32

Summary:
In the same manner as struct.buffer.load / struct.buffer.store,
allow struct.buffer.load.format / struct.buffer.store.format to
return / accept any type.  This simplifies front-end code gen.

Reviewers: tpr, arsenm, nhaehnle

Reviewed By: arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75789