i386: Fix emit_reduc_half on V{64Q,32H}Imode [PR94500]

i386: Fix emit_reduc_half on V{64Q,32H}Imode [PR94500]

The following testcase is miscompiled in 8.x, because emit_reduc_half is
prepared to handle for 512-bit modes only i equal to 512, 256, 128 and 64.
V32HImode also needs i equal to 32 and V64QImode i equal to 32 and 16,
but emit_reduc_half in that case performs a redundant permutation exactly
like i == 32.  In 9+ the testcase works because Richard in r9-3393
changed the reduc_* expanders so that they actually don't call
ix86_expand_reduc on 512-bit modes, but only 128-bit ones.

The patch fixes emit_reduc_half to handle also i of 32 and 16 similarly to
how V32QImode/V16HImode are handled for AVX2.  I think it shouldn't hurt
to fix the function even on the trunk and 9 branch even when nothing uses
it ATM.

2020-04-07  Jakub Jelinek  <jakub@redhat.com>

	PR target/94500
	* config/i386/i386-expand.c (emit_reduc_half): For V{64QI,32HI}mode
	handle i < 64 using avx512bw_lshrv4ti3.  Formatting fixes.

	* gcc.target/i386/avx512bw-pr94500.c: New test.