[RISCV][GISel] Make G_CTLZ legal with XTHeadBb (#237510)
XTHeadBb provides `th.ff1` and matches `G_CTLZ` semantics. The existing
pattern `PatGpr<ctlz, TH_FF1>` is already imported by GlobalISel, but
the legalizer always lowered `G_CTLZ` when Zbb was not available, so
`th.ff1` was never selected.
Part of https://github.com/llvm/llvm-project/issues/237492.
I plan to address the whole problem with 3 patches to control patch size
and ease review process:
1. This commit
2. Use the zero-poison semantics of `G_CTLZ_ZERO_POISON` for narrower
widths.
3. Widen narrow `G_CTTZ` to XLEN before lowering.
