arch-gcn3: fix bits that SDWA selects

arch-gcn3: fix bits that SDWA selects

This commit fixes a bug in 200f2408 where the SDWA support was selecting bits
backwards.  As part of this commit, to help resolve this problem in the
future, I have added asserts in the helper functions in bitfield.hh to ensure
that the number of bits aren't negative.

Change-Id: I4b0ecb0e7c110600c0b5063101b75f9adcc512ac
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29931
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>