cmd/compile: optimize switch statements using lookup tables

Switch statement containing integer constant cases and case bodies just
returning a constant should be optimizable to a simpler and faster table
lookup instead of a jump table.

That is, a switch like this:

    switch x {
    case 0: return 10
    case 1: return 20
    case 2: return 30
    case 3: return 40
    default: return -1
    }

Could be optimized to this:

    var table = [4]int{10, 20, 30, 40}
    if uint(x) < 4 { return table[x] }
    return -1

The resulting code is smaller and faster, especially on platforms where
jump tables are not supported.

goos: windows
goarch: arm64
pkg: cmd/compile/internal/test
                               │  .\old.txt  │              .\new.txt              │
                               │   sec/op    │   sec/op     vs base                │
SwitchLookup8Predictable-12      2.708n ± 6%   2.249n ± 5%  -16.97% (p=0.000 n=10)
SwitchLookup8Unpredictable-12    8.758n ± 7%   3.272n ± 4%  -62.65% (p=0.000 n=10)
SwitchLookup32Predictable-12     2.672n ± 5%   2.373n ± 6%  -11.21% (p=0.000 n=10)
SwitchLookup32Unpredictable-12   9.372n ± 7%   3.385n ± 6%  -63.89% (p=0.000 n=10)
geomean                          4.937n        2.772n       -43.84%

Fixes #78203

Change-Id: I74fa3d77ef618412951b2e5c3cb6ebc760ce4ff1
Reviewed-on: https://go-review.googlesource.com/c/go/+/756340
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>