Fix the recently added anyregcc convention to handle spilled operands.

Fixes <rdar://15432754> [JS] Assertion: "Folded a def to a non-store!"

The primary purpose of anyregcc is to prevent a patchpoint's call
arguments and return value from being spilled. They must be available
in a register, although the calling convention does not pin the
register. It's up to the front end to avoid using this convention for
calls with more arguments than allocatable registers.

llvm-svn: 194428