mem-ruby: Fixing token port responses in GPUCoalescer

mem-ruby: Fixing token port responses in GPUCoalescer

The is a bug in the GPUCoalescer which occurs in the following
situation:

1) An instruction crosses a page boundary causing multiple TLB requests
to be sent.
2) The TLB responses arrive at different times, causing the vector
memory requests to be sent at different times.
3) The first vector memory request completes before the second vector
memory request arrives at the coalescer.

This caused the coalescer to consider the instruction sequence number
done and return its token. Then the second request would arrive and
complete sending back another token. Eventually this increases the token
count beyond the maximum tripping an assert.

This change keeps track of the number of per-lane requests which are
expected to be sent in the vector memory request by looking at the exec
mask of the instruction. The token is not returned until the expected
number of per-lane requests have been coalesced. This fixes "#7" in the
list of issues in JIRA-300. There are also style fixes for local
variables in code nearby the changes in this CL.

Change-Id: I152fd9397920ad82ba6079112908387e71ff3cce
JIRA: https://gem5.atlassian.net/browse/GEM5-300
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35176
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Kyle Roarty <kyleroarty1716@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>