Teach VN about masked/len stores

The following teaches VN to handle reads from .MASK_STORE and
.LEN_STORE.  For this push_partial_def is extended first for
convenience so we don't have to handle the full def case in the
caller (possibly other paths can be simplified then).  Also
the partial definition stored value can have an offset applied
so we don't have to build a fake RHS when we register the pieces
of an existing store.

	PR tree-optimization/106365
	* tree-ssa-sccvn.cc (pd_data::rhs_off): New field determining
	the offset to start encoding of RHS from.
	(vn_walk_cb_data::vn_walk_cb_data): Initialize it.
	(vn_walk_cb_data::push_partial_def): Allow the first partial
	definition to be fully providing the def.  Offset RHS
	before encoding if requested.
	(vn_reference_lookup_3): Initialize def_rhs everywhere.
	Add support for .MASK_STORE and .LEN_STORE (partial) definitions.

	* gcc.target/i386/vec-maskstore-vn.c: New testcase.

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