gallivm: fix SCALED -> NORM conversions

gallivm: fix SCALED -> NORM conversions

Such conversions (which are most likely rather pointless in practice) were
resulting in shifts with negative shift counts and shifts with counts the same
as the bit width. This was always undefined in llvm, the code generated was
rather horrendous but happened to work.
So make sure such shifts are filtered out and replaced with something that
works (the generated code is still just as horrendous as before).

This fixes lp_test_format, https://bugs.freedesktop.org/show_bug.cgi?id=73846.

v2: prettify by using build context shift helpers.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>