mem-cache: Avoid write merging if there are reads in between

mem-cache: Avoid write merging if there are reads in between

This CL reworks the logic in the MSHR to make sure we do not coalesce
requests unless there is a series of write requests for the whole
cache block without any other incompatible requests (e.g., read) in
between.

Change-Id: I0b3195858fb33ef85d7aae27376506057dd53ea7
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23666
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Tested-by: kokoro <noreply+kokoro@google.com>