codegen: check for PPC64 ISEL in condmove tests
codegen: check for PPC64 ISEL in condmove tests

ISEL is roughly equivalent to CMOV on PPC64. Verify ISEL generation
in all reasonable cases.

Note "ISEL test x y z" is the same as "ISEL !test y x z". test is
always one of LT (0), GT (1), EQ (2), SO (3). Sometimes x and y are
swapped if GE/LE/NE is desired.

Change-Id: Ie1bf029224064e004d855099731fe5e8d05aa990
Reviewed-on: https://go-review.googlesource.com/c/go/+/445215
Reviewed-by: Lynn Boger <b27445fe6fb259c37b8e0a4ce56e22e2413754bf@linux.vnet.ibm.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Paul Murphy <2e1d8d6f81223fe22f04c5349bc2b02555a275af@ibm.com>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go