runtime: initialize itab.hash always
We weren't initializing this field for dynamically-generated itabs.
Turns out it doesn't matter, as any time we use this field we also
generate a static itab for the interface type / concrete type pair.
But we should initialize it anyway, just to be safe.
Performance on the benchmarks in CL 44339:
benchmark old ns/op new ns/op delta
BenchmarkItabFew-12
1040585 26466 -97.46%
BenchmarkItabAll-12
228873499 4287696 -98.13%
Change-Id: I58ed2b31e6c98b584122bdaf844fee7268b58295
Reviewed-on: https://go-review.googlesource.com/44475
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>