[AMDGPU] NFC refactoring in isel for buffer access intrinsics

Rename getBufferOffsetForMMO to updateBufferMMO and pass in the MMO to
be updated, in preparation for the bug fix in D106284.

Call updateBufferMMO consistently for all buffer intrinsics, even the
ones that use setBufferOffsets to decompose a combined offset
expression.

Add a getIdxEn helper function.

Differential Revision: https://reviews.llvm.org/D106354