cmd/compile: eliminate redundant sign-extensions for wasm

Add rules to eliminate redundant I64Extend sign-extension operations
in the wasm backend:

Idempotent (applying the same extend twice is redundant):

  (I64Extend32S (I64Extend32S x)) => (I64Extend32S x)
  (I64Extend16S (I64Extend16S x)) => (I64Extend16S x)
  (I64Extend8S  (I64Extend8S  x)) => (I64Extend8S  x)

Narrower-subsumes-wider (a narrower sign-extend already determines
all the bits that a wider one would set):

  (I64Extend32S (I64Extend16S x)) => (I64Extend16S x)
  (I64Extend32S (I64Extend8S  x)) => (I64Extend8S  x)
  (I64Extend16S (I64Extend8S  x)) => (I64Extend8S  x)

These patterns arise from nested sub-word type conversions. For
example, converting int8 -> int16 -> int32 -> int64 lowers to
I64Extend8S -> I64Extend16S -> I64Extend32S, but the I64Extend8S
alone is sufficient since it already sign-extends from 8 to 64 bits.

Cq-Include-Trybots: luci.golang.try:gotip-wasip1-wasm_wasmtime,gotip-wasip1-wasm_wazero
Change-Id: I1637687df31893b1ffa36915a3bd2e10d455f4ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/754040
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

0 files changed, 0 insertions, 0 deletions