s390x: Fix popcounthi2_z196 expander [PR93533]

s390x: Fix popcounthi2_z196 expander [PR93533]

The following testcase started to ICE when .POPCOUNT matching has been added
to match.pd; we had __builtin_popcount*, but nothing would use the
popcounthi2 expander before.

The problem is that the popcounthi2_z196 expander doesn't emit valid RTL:
error: unrecognizable insn:
(insn 138 137 139 27 (set (reg:SI 190)
        (ashift:SI (reg:HI 95 [ _105 ])
            (const_int 8 [0x8]))) -1
     (nil))
during RTL pass: vregs
The following patch is an attempt to fix that, furthermore I've tried to
slightly simplify it as well, it makes no sense to me to perform
(x + (x << 8)) >> 8 when we need to either zero extend or mask the result
at the end in order to avoid bits from above HImode to affect it, when we
can do
(x + (x >> 8)) & 0xff (or zero extension).

2020-02-03  Jakub Jelinek  <jakub@redhat.com>

	PR target/93533
	* config/s390/s390.md (popcounthi2_z196): Fix up expander to emit
	valid RTL to sum up the lowest and second lowest bytes of the popcnt
	result.

	* gcc.c-torture/compile/pr93533.c: New test.
	* gcc.target/s390/pr93533.c: New test.