[MemRefToLLVM] Fix the lowering of memref.assume_alignment

`memref.assume_alignment` annotates the alignment of the source buffer
not the base pointer.
Put diffrently, prior to this patch `memref.assume_alignment` would lower
to `llvm.assume %buffer.base.isAligned(X)` whereas what we want is
`llvm.assume (%buffer.base + %buffer.offset).isAligned(X)`.
In other words, we were missing to include the offset in the expression
checked by the `llvm.assume`.

Differential Revision: https://reviews.llvm.org/D148930