optimise LSBit mask (followed by branch) to use `ctz`

[ggreif]

I have code snippets like

i32.const 1
i32.and
i32.eqz
br_if 1 (;@1;)

which could be optimised to

i32.ctz
br_if 1 (;@1;)

Similar analogous transformations:

• i32.const 1; i32.and; i32.eqz; if to i32.ctz; if
• i32.const 1; i32.and; if to i32.ctz; if (with legs swapped)
• possibly 64-bit variants (but here probably instructions for truncation need to be added)
• what about sightedness testing? (cmp against 0 --> clz)