cmd/compile: add saturating conversions on wasm

cmd/compile: add saturating conversions on wasm

This change adds the GOWASM option "satconv" to enable the generation
of experimental saturating (non-trapping) float-to-int conversions.
It improves the performance of the conversion by 42%.

Previously the conversions had already been augmented with helper
functions to have saturating behavior. Now Wasm.rules is always using
the new operation names and wasm/ssa.go is falling back to the helpers
if the feature is not enabled.

The feature is in phase 4 of the WebAssembly proposal process:
https://github.com/WebAssembly/meetings/blob/master/process/phases.md

More information on the feature can be found at:
https://github.com/WebAssembly/nontrapping-float-to-int-conversions/blob/master/proposals/nontrapping-float-to-int-conversion/Overview.md

Change-Id: Ic6c3688017054ede804b02b6b0ffd4a02ef33ad7
Reviewed-on: https://go-review.googlesource.com/c/go/+/170119
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>