cmd/compile/internal/walk: remove reduceSlice
cmd/compile/internal/walk: remove reduceSlice

After CL 22425, there're two optimizations for slice expr which are
never applied during walk pass:

	s[i:len(s)]
        s[i:j:cap(s)]

The order pass have already rewritten len/cap expression to use autotmp,
thus the same safe expression check will never fire. The code can now be
simplified by moving the only case left from reduceSlice to walkSlice,
then removing reduceSlice entirely.

Passes toolstash-check.

Change-Id: Ia8cfb15c8e96c186a214c17b42d0fee51b0d3a1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/432695
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Auto-Submit: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@google.com>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go