PR c++/55095
	* c-common.c (maybe_warn_shift_overflow): Properly handle
	left-shifting 1 into the sign bit.

	* c-c++-common/Wshift-overflow-6.c: New test.
	* c-c++-common/Wshift-overflow-7.c: New test.
	* g++.dg/cpp1y/left-shift-2.C: New test.

From-SVN: r226826