[PowerPC] Add FI alignment check if the addressing mode is DS/DQ-Form, emit X-Form...

[PowerPC] Add FI alignment check if the addressing mode is DS/DQ-Form, emit X-Form if necessary.

This patch adds a function that checks whether or not the frame index
is aligned when the computed addressing mode is an aligned D-Form (DS, or DQ-Form).
If the frame index appears to be unaligned, within these two modes, reset
the mode to X-Form in order to fall back to selection X-Form loads.

A test case is added to ensure that the test emits X-Form loads and not DQ-Form
loads since the frame index is not aligned within the test case.

Differential Revision: https://reviews.llvm.org/D105661