Teach fast isel about the win64 calling convention.

This mostly just works.

Vectorcall rets are still not supported.

The win64_eh test change is because fast isel doesn't use rsi for temporary
computations, so it doesn't need to be pushed. The test case I'm changing was
originally added to test pushes, but by now there are other test cases in that
file exercising that code path.

https://reviews.llvm.org/D22422

llvm-svn: 275607