tcg/optimize: Optimize sign extensions

Certain targets, like riscv, produce signed 32-bit results.
This can lead to lots of redundant extensions as values are
manipulated.

Begin by tracking only the obvious sign-extensions, and
converting them to simple copies when possible.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>