[GlobalISel] Optimize ULEB128 usage (#90565)

- Remove some cases where ULEB128 isn't needed
- Add a fastDecodeULEB128 tailored for GlobalISel which does unchecked
decoding optimized for the common case, which is 1 byte values. We
rarely have >1 byte Inst IDs, OpIdx, etc. and those are the most common
ULEB users by far.

This specific LEB128 decode function generates almost 2x less
instructions than the generic one.