This patch tweaks the match.pd transformation previously added to fold
(X<<C)+(Y<<C) as (X+Y)<<C that was previously restricted to unsigned
(wrapping) types, to also allow signed integer types provided that they
don't trap and the overflow needn't be preserved for sanitization.
i.e. this should now apply (by default) for "int x,y;", but is disabled
with -ftrapv.

This unsigned transformation has baked on mainline without problems, so it's
time to turn up the heat...  LLVM, icc and MSVC perform this optimization.


This patch has been tested on x86_64-pc-linux-gnu with make bootstrap and
make -k check, both with and without --target_board=unix{-m32}, with no
new failures.  Ok for mainline?


2022-09-13  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * match.pd (op (lshift @0 @1) (lshift @2 @1)): Optimize the
        expression (X<<C) + (Y<<C) to (X+Y)<<C for signed types, that
        may have undefined overflow, provided that they don't trap.

gcc/testsuite/ChangeLog
        * gcc.dg/pr71343-3.c: New test case.


Thanks in advance,
Roger
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