match.pd: Optimize sign-extension followed by truncation [PR113024]

match.pd: Optimize sign-extension followed by truncation [PR113024]

While looking at a bitint ICE, I've noticed we don't optimize
in f1 and f5 functions below the 2 casts into just one at GIMPLE,
even when optimize it in convert_to_integer if it appears in the same
stmt.  The large match.pd simplification of two conversions in a row
has many complex rules and as the testcase shows, everything else from
the narrowest -> widest -> prec_in_between all integer conversions
is already handled, either because the inside_unsignedp == inter_unsignedp
rule kicks in, or the
         && ((inter_unsignedp && inter_prec > inside_prec)
             == (final_unsignedp && final_prec > inter_prec))
one, but there is no reason why sign extension to from narrowest to
widest type followed by truncation to something in between can't be
done just as sign extension from narrowest to the final type.  After all,
if the widest type is signed rather than unsigned, regardless of the final
type signedness we already handle it that way.
And since PR93044 we also handle it if the final precision is not wider
than the inside precision.

2023-12-15  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/113024
	* match.pd (two conversions in a row): Simplify scalar integer
	sign-extension followed by truncation.

	* gcc.dg/tree-ssa/pr113024.c: New test.