middle-end/112740 - vector boolean CTOR expansion issue

The optimization to expand uniform boolean vectors by sign-extension
works only for dense masks but it failed to check that.

	PR middle-end/112740
	* expr.cc (store_constructor): Check the integer vector
	mask has a single bit per element before using sign-extension
	to expand an uniform vector.

	* gcc.dg/pr112740.c: New testcase.