arch-gcn3: Read registers in execute instead of initiateAcc
arch-gcn3: Read registers in execute instead of initiateAcc

Certain memory writes were reading their registers in
initiateAcc, which lead to scenarios where a subsequent instruction
would execute, clobbering the value in that register before the memory
writes' initiateAcc method was called, causing the memory write to read
wrong data.

This patch moves all register reads to execute, preventing the above
scenario from happening.

Change-Id: Iee107c19e4b82c2e172bf2d6cc95b79983a43d83
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45345
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Matt Sinclair <d2406197fb1bedd1e23c70c50c510ff0e9ab0f5b@gmail.com>
Reviewed-by: Matthew Poremba <f1eb50f3368ce197c52b19f7fdb5aa7a278f15fd@amd.com>
Reviewed-by: Alex Dutu <15b44ee77720049b2fb3caf7ffa01fa902c6849f@amd.com>
Maintainer: Matt Sinclair <d2406197fb1bedd1e23c70c50c510ff0e9ab0f5b@gmail.com>

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