[PowerPC] Support multiple return values with fast isel

Using an LLVM IR aggregate return value type containing three
or more integer values causes an abort in the fast isel pass.

This patch adds two more registers to RetCC_PPC64_ELF_FIS to
allow returning up to four integers with fast isel, just the
same as is currently supported with regular isel (RetCC_PPC).

This is needed for Swift and (possibly) other non-clang frontends.

Fixes PR26190.

llvm-svn: 272005