[mlir][AMDGPU] Explicitly truncate memory addresses in buffer ops

As a percaution, truncate memory addresses passed to kernels to 48 bits,
since bits 48-63 of the buffer descriptor are used for the stride field
and, on gfx10, to control swizzling.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D131016