Change behavior of calling bitcasted alias functions.

It will now only convert the arguments / return value and call
the underlying function if the types are able to be bitcasted.
This avoids using fp<->int conversions that would occur before.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187444 91177308-0d34-0410-b5e6-96231b3b80d8