arch-gcn3: Fix V_MAD_I32_I24 sign extension

arch-gcn3: Fix V_MAD_I32_I24 sign extension

We are not properly sign extending the bits we hack off for
V_MAD_I32_I24.

This fixes rnn_fwdBwd 64 1 1 lstm pte assertion failure.

Change-Id: I2516e5715227cbd822e6a62630674f64f7a109e0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29928
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>