[WebAssembly] Don't assume that zext/sext result is i32/i64 in fast isel (PR41841)

Usually this will abort fast-isel at the instruction using the
non-legal result, but if the only use is in a different basic block,
we'll incorrectly assume that the zext/sext is to i32 (rather than
i128 in this case).

Differential Revision: https://reviews.llvm.org/D61823

llvm-svn: 360616