[PowerPC] Fix EmitPPCBuiltinExpr to emit arguments once

This patch changes `EmitPPCBuiltinExpr` in `CGBuiltin.cpp` to remove
the loop at the beginning of the function that emits the arguments and
to delay emitting the arguments until inside the switch statement. These
changes will put `EmitPPCBuiltinExpr` in line with the strategy of the
target independent function `EmitBuiltinExpr`. Also, this patch
ensures that arguments are only emitted once.

Tests that included builtins affected by these changes have been
modified to match expected behaviour.

Reviewed By: #powerpc, nemanjai, amyk

Differential Revision: https://reviews.llvm.org/D121637