target/120234 - adjust vector construction costs

The following allows vectorizing the gcc.target/i386/pr111023*.c
testcases again with -m32 -msse2 by ensuring we see through a cast
when looking for vector extract sources during costing of vector construction.

This, together with the forwprop fix fixes the regression on those testcases.

	PR target/120234
	* config/i386/i386.cc (ix86_vector_costs::add_stmt_cost):
	For constructor elements always look through a conversion.
	Rewrite load and vector extraction matching to be more obvious.
	Allow arbitrary conversions from the vector extract to elide
	costing of a gpr<->xmm move.