cmd/compile: improve store-to-load forwarding with compatible types

cmd/compile: improve store-to-load forwarding with compatible types

Improve the compiler's store-to-load forwarding optimization by relaxing the
type comparison condition. Instead of requiring exact type equality (CMPeq),
we now use copyCompatibleType which allows forwarding between compatible
types where safe.

Fix several size comparison bugs in the nested store patterns. Previously,
we were comparing the size of the outer store with the load type,
rather than comparing with the size of the actual store being forwarded
from.

Skip OpConvert in dead store elimination to help get rid of dead stores such
as zeroing slices. OpConvert, like OpInlMark, doesn't really use the memory.

This optimization is particularly beneficial for code that creates slices with
computed pointers, such as the runtime's heapBitsSlice function, where
intermediate calculations were previously causing the compiler to miss
store-to-load forwarding opportunities.

Local sweet run result on an x86_64 laptop:

                       │  Orig.res   │              Hopt.res              │
                       │   sec/op    │   sec/op     vs base               │
BiogoIgor-8               5.303 ± 1%    5.322 ± 1%       ~ (p=0.190 n=10)
BiogoKrishna-8            7.894 ± 1%    7.828 ± 2%       ~ (p=0.190 n=10)
BleveIndexBatch100-8      2.257 ± 1%    2.248 ± 2%       ~ (p=0.529 n=10)
EtcdPut-8                30.12m ± 1%   30.03m ± 1%       ~ (p=0.796 n=10)
EtcdSTM-8                127.1m ± 1%   126.2m ± 0%  -0.74% (p=0.023 n=10)
GoBuildKubelet-8          52.21 ± 0%    52.05 ± 1%       ~ (p=0.063 n=10)
GoBuildKubeletLink-8      4.342 ± 1%    4.305 ± 0%  -0.85% (p=0.000 n=10)
GoBuildIstioctl-8         43.33 ± 0%    43.24 ± 0%  -0.22% (p=0.015 n=10)
GoBuildIstioctlLink-8     4.604 ± 1%    4.598 ± 0%       ~ (p=0.063 n=10)
GoBuildFrontend-8         15.33 ± 0%    15.29 ± 0%       ~ (p=0.143 n=10)
GoBuildFrontendLink-8    740.0m ± 1%   737.7m ± 1%       ~ (p=0.912 n=10)
GopherLuaKNucleotide-8    9.590 ± 1%    9.656 ± 1%       ~ (p=0.165 n=10)
MarkdownRenderXHTML-8    96.97m ± 1%   97.26m ± 2%       ~ (p=0.105 n=10)
Tile38QueryLoad-8        335.9µ ± 1%   335.6µ ± 1%       ~ (p=0.481 n=10)
geomean                   1.336         1.333       -0.22%

Change-Id: I031552623e6d5a3b1b5be8325e6314706e45534f
Reviewed-on: https://go-review.googlesource.com/c/go/+/662075
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@golang.org>