[Libre-soc-dev] SimdSignal update

[Jacob Lifshay]

I ended up somewhat distracted by other stuff, then I finished figuring out
all of what you have done so far for SimdSignal and Cat, but by then it was
rather late and Cat would likely take time, so I instead worked on adding
fixed_width tracking to XLEN and adding the missing tests for util.py.

Since XLEN-based arithmetic may end up with a
inconsistent/garbage/non-existant value for fixed_width, I decided to name
it SimdWHintMap.width_hint instead.

The idea is that SimdShape will check if width_hint looks sane and, if not,
ignore it and use the width calculated from element widths like usual.

Jacob