tree-optimization/121405 - missed VN with aggregate copy

tree-optimization/121405 - missed VN with aggregate copy

The following handles value-numbering of a BIT_FIELD_REF of
a register that's defined by a load by looking up a subset
load similar to how we handle bit-and masked loads.  This
allows the testcase to be simplified by two FRE passes,
the first one will create the BIT_FIELD_REF.

	PR tree-optimization/121405
	* tree-ssa-sccvn.cc (visit_nary_op): Handle BIT_FIELD_REF
	with reference def by looking up a combination of both.

	* gcc.dg/tree-ssa/ssa-fre-107.c: New testcase.
	* gcc.target/i386/pr90579.c: Adjust.