[WebAssembly] Fix FastISel of condition in different block (PR51651)

If the icmp is in a different block, then the register for the icmp
operand may not be initialized, as it nominally does not have
cross-block uses. Add a check that the icmp is in the same block
as the branch, which should be the common case.

This matches what X86 FastISel does:
https://github.com/llvm/llvm-project/blob/5b6b090cf2129228f05d7d0f504676b67f7524cf/llvm/lib/Target/X86/X86FastISel.cpp#L1648

The "not" transform that could have a similar issue is dropped
entirely, because it is currently dead: The incoming value is
a branch or select condition of type i1, but this code requires
an i32 to trigger.

Fixes https://bugs.llvm.org/show_bug.cgi?id=51651.

Differential Revision: https://reviews.llvm.org/D108840