fix SKVM_ benches
fix SKVM_ benches

Things were running suspiciously well...

_I32 had a typo that cut out 3/4 of its multiplies...

_I32_SWAR was missing a mask operation needed to drop
the junk low byte of the high half after the multiply.

The bench times now make a bit more sense and are in line
with how much work we're actually doing: F32's the slowest,
I32 a little faster, and I32_SWAR fastest:

    curr/maxrss	loops	min	median	mean	max	stddev	samples   	config	bench
      35/36  MB	58	2.03ns	2.04ns	2.04ns	2.04ns	0%	▂▂▂▂▁▁█▁▂▁	nonrendering	SkVM_4096_I32_SWAR
      35/36  MB	42	3.44ns	3.48ns	3.49ns	3.59ns	1%	▂▆▅█▃▃▁▂▂▄	nonrendering	SkVM_4096_I32
      35/36  MB	30	4.9ns	5.21ns	5.11ns	5.33ns	3%	▆▇█▆▆▁▂▁▁▅	nonrendering	SkVM_4096_F32
      35/36  MB	203	0.696ns	0.697ns	0.705ns	0.758ns	3%	█▂▂▁▁▁▁▁▁▂	nonrendering	SkVM_4096_RP
      35/36  MB	942	0.188ns	0.188ns	0.188ns	0.189ns	0%	▂▁▂▁▃█▂▁▁▁	nonrendering	SkVM_4096_Opts

Change-Id: I2850dc3f9df1828f03499eb278b8231f48eaae63
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/217982
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Auto-Submit: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

google/skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia