[AArch64] Lower extending uitofp using tbl.

On AArch64, doing the zero-extend separately first can be lowered more
efficiently using tbl, building on D120571.

https://alive2.llvm.org/ce/z/8Je595

Depends on D120571

Reviewed By: t.p.northover

Differential Revision: https://reviews.llvm.org/D133494