tree-optimization/92819 restrict new vector CTOR canonicalization

tree-optimization/92819 restrict new vector CTOR canonicalization

The PR shows that code generation ends up pessimized by the new
canonicalization rules that end up nailing do-not-care elements
to specific values making it hard to generate good code later.

The temporary solution is to avoid this for the cases we also
obviously know the canonicalization will create more GIMPLE stmts than
before.

2020-02-04  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/92819
	* tree-ssa-forwprop.c (simplify_vector_constructor): Avoid
	generating more stmts than before.

	* gcc.target/i386/pr92819.c: New testcase.
	* gcc.target/i386/pr92803.c: Adjust.