cmd/compile: wire up math/bits.Len intrinsics for loong64

cmd/compile: wire up math/bits.Len intrinsics for loong64

For the SubFromLen64 codegen test case to work as intended, we need
to fold c-(-(x-d)) into x+(c-d).

Still, some instances of LeadingZeros are not optimized into single
CLZ instructions right now (actually, the LeadingZeros micro-benchmarks
are currently still compiled with redundant adds/subs of 64, due to
interference of loop optimizations before lowering), but perf numbers
indicate it's not that bad after all.

Micro-benchmark results on Loongson 3A5000 and 3A6000:

goos: linux
goarch: loong64
pkg: math/bits
cpu: Loongson-3A5000 @ 2500.00MHz
               |  bench.old  |              bench.new              |
               |   sec/op    |   sec/op     vs base                |
LeadingZeros     3.660n ± 0%   1.348n ± 0%  -63.17% (p=0.000 n=20)
LeadingZeros8    1.777n ± 0%   1.767n ± 0%   -0.56% (p=0.000 n=20)
LeadingZeros16   2.816n ± 0%   1.770n ± 0%  -37.14% (p=0.000 n=20)
LeadingZeros32   5.293n ± 1%   1.683n ± 0%  -68.21% (p=0.000 n=20)
LeadingZeros64   3.622n ± 0%   1.349n ± 0%  -62.76% (p=0.000 n=20)
geomean          3.229n        1.571n       -51.35%

goos: linux
goarch: loong64
pkg: math/bits
cpu: Loongson-3A6000 @ 2500.00MHz
               |  bench.old   |              bench.new               |
               |    sec/op    |    sec/op     vs base                |
LeadingZeros      2.410n ± 0%    1.103n ± 1%  -54.23% (p=0.000 n=20)
LeadingZeros8     1.236n ± 0%    1.501n ± 0%  +21.44% (p=0.000 n=20)
LeadingZeros16    2.106n ± 0%    1.501n ± 0%  -28.73% (p=0.000 n=20)
LeadingZeros32    2.860n ± 0%    1.324n ± 0%  -53.72% (p=0.000 n=20)
LeadingZeros64   2.6135n ± 0%   0.9509n ± 0%  -63.62% (p=0.000 n=20)
geomean           2.159n         1.256n       -41.81%

Updates #59120

This patch is a copy of CL 483356.
Co-authored-by: WANG Xuerui <git@xen0n.name>
Change-Id: Iee81a17f7da06d77a427e73dfcc016f2b15ae556
Reviewed-on: https://go-review.googlesource.com/c/go/+/624575
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>