compiler: remove ArchWasm32 special case in cmmDoCmmSwitchPlans

This patch removes special consideration for ArchWasm32 in
cmmDoCmmSwitchPlans, which means the compiler will now disable
cmmImplementSwitchPlans for wasm unreg backend, just like unreg
backend of other targets. We enabled it in the past to workaround some
compile-time panic in older versions of LLVM, but those panics are no
longer present, hence no need to keep this workaround.

(cherry picked from commit bf0737c0)
(cherry picked from commit 8bd28cf2)