cmd/compile: make isConstDelta compute delta for 8 and 16 bit arithmetic

cmd/compile: make isConstDelta compute delta for 8 and 16 bit arithmetic

Use the delta for computing min and max values. This elides a few bounds checks:

compilecmp linux/amd64:

regexp/syntax
regexp/syntax.Op.String 271 -> 249  (-8.12%)

compress/bzip2
compress/bzip2.(*reader).readBlock 2991 -> 2973  (-0.60%)

cmd/internal/objabi
cmd/internal/objabi.RelocType.String 240 -> 220  (-8.33%)

cmd/vendor/golang.org/x/arch/ppc64/ppc64asm
cmd/vendor/golang.org/x/arch/ppc64/ppc64asm.CondReg.String 421 -> 400  (-4.99%)
cmd/vendor/golang.org/x/arch/ppc64/ppc64asm.gnuArg changed
cmd/vendor/golang.org/x/arch/ppc64/ppc64asm.plan9Arg 1868 -> 1836  (-1.71%)

cmd/internal/objfile
cmd/internal/objfile.(*machoFile).symbols 1457 -> 1423  (-2.33%)
cmd/internal/objfile.loadPETable changed

cmd/internal/obj/wasm
cmd/internal/obj/wasm.assemble changed

cmd/internal/obj/ppc64
cmd/internal/obj/ppc64.type_vsrdbi changed
cmd/internal/obj/ppc64.type_vmsumcud changed

cmd/link/internal/loadpe
cmd/link/internal/loadpe.Load 10634 -> 10602  (-0.30%)
cmd/link/internal/loadpe.(*peLoaderState).readpesym changed

Change-Id: I439facd13e3d2695abadfe1d3f7faebfd0d7df74
Reviewed-on: https://go-review.googlesource.com/c/go/+/431237
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>