Rewrite power-of-two indexed word selects to $bmux when the shift
amount already carries the scale as low zero bits.
Keep the rule to non-overlapping selections and bound the generated
mux ways. Add regressions for aligned shifts, padding, signed
extension, and shiftmul handoff cases.
- Techlib pmgens are now in relevant techlibs/*.
- `peepopt` pmgens are now in passes/opt.
- `test_pmgen` is still in passes/pmgen.
- Update `Makefile.inc` and `.gitignore` file(s) to match new `*_pm.h` location,
as well as the `#include`s.
- Change default `%_pm.h` make target to `techlibs/%_pm.h` and move it to the
top level Makefile.
- Update pmgen target to use `$(notdir $*)` (where `$*` is the part of the file
name that matched the '%' in the target) instead of `$(subst _pm.h,,$(notdir
$@))`.
