tree-optimization: [PR103218] Fold ((type)(a<0)) << SIGNBITOFA into ((type)a) & signbit

This folds Fold ((type)(a<0)) << SIGNBITOFA into ((type)a) & signbit inside match.pd.
This was already handled in fold-cost by:
/* A < 0 ? <sign bit of A> : 0 is simply (A & <sign bit of A>).  */
I have not removed as we only simplify "a ? POW2 : 0" at the gimple level to "a << CST1"
and fold actually does the reverse of folding "(a<0)<<CST" into "(a<0) ? 1<<CST : 0".

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

	PR tree-optimization/103218

gcc/ChangeLog:

	* match.pd: New pattern for "((type)(a<0)) << SIGNBITOFA".

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr103218-1.c: New test.