This adds code to protect WebAssembly's `trunc_s` family of opcodes
from values outside their domain. Even though such conversions have
full undefined behavior in C/C++, LLVM IR's `fptosi` and `fptoui` do
not, and only return undef.
This also implements the proposed non-trapping float-to-int conversion
feature and uses that instead when available.
llvm-svn: 319128