all: rename WebAssembly instructions according to spec changes

The names of some instructions have been updated in the WebAssembly
specification to be more consistent, see
https://github.com/WebAssembly/spec/commit/994591e51c9df9e7ef980b04d660709b79982f75.
This change to the spec is possible because it is still in a draft
state.

Go's support for WebAssembly is still experimental and thus excempt from
the compatibility promise. Being consistent with the spec should
warrant this breaking change to the assembly instruction names.

Change-Id: Iafb8b18ee7f55dd0e23c6c7824aa1fad43117ef1
Reviewed-on: https://go-review.googlesource.com/c/153797


Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>