[AArch64][GISel] Legalize non-power-of-two G_CTTZ

The main change here is to add a `widenScalarToNextPow2` before the
`clampScalar` so that non-power-of-two sizes between 32 and 64 are
turned into s64 count trailing zeroes.

However, if you make the legalisation rules depend on TypeIdx 0 (the
output), then you still get crashes for the s65 testcase, which I solved
by instead flipping the rules around to be about TypeIdx 1 (the input),
with a `scalarSameSizeAs` at the end to tie index 0 to index 1. This,
incidentally, is how things are written for `G_CTLZ`.

Differential Revision: https://reviews.llvm.org/D147602