[AMDGPU][GFX908] IndirectCopyToAGPR: Confirm modified register is dst reg of accvgpr_write

IndirectCopyToAGPR should be reworked as to avoid optimizing during copy lowering. However, as it stands, the code is buggy. This patch replaces the call to definesRegister with modifiesRegister, and confirms that the dest reg of the found accvgpr_write is in fact the src reg of our copy.

Differential Revision: https://reviews.llvm.org/D149873

Change-Id: Id8a61659ac15565dcb970069d0624f0925a46e6d