cmd/compile: unnecessary bounds check when indexing relative to the end

[dsnet]

Using go1.17.5

Consider the following:

len(b) >= 3 && b[len(b)-3] == '#'

Part of the assembly includes:

0x002f 00047 (main.go:6)	CALL	runtime.panicIndex(SB)

This should be unnecessary since the previous len(b) >= 3 check ensures that the buffer must contain the len(b)-3 element. Thus, there is no possibility of the index panicking.

\cc @martisch @randall77

[gopherbot]

Change https://go.dev/cl/406175 mentions this issue: cmd/compile: derive relation between x+delta and x in prove

[gopherbot]

Change https://go.dev/cl/402815 mentions this issue: cmd/compile: Rules to prove for better bound check elimination