Revert "powerpc: slightly improve cache helpers"

This reverts commit 6c587584

.

It triggers a probable compiler bug on clang which leads to crashes.

With GCC it allows the compiler to use a more efficient register
allocation but current GCC versions never do that at any of the current
call sites, so there's no benefit.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>