[InstCombine] Do not introduce bitcasts for swifterror arguments.

The following constraints hold for swifterror values:

    A swifterror value (either the parameter or the alloca) can only
    be loaded and stored from, or used as a swifterror argument.

This patch updates instcombine to not try to convert a bitcast of a
function into a bitcast of a swifterror argument.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D90258