SCCVN: Fix repeating variable name "len"

SCCVN: Fix repeating variable name "len"

Line 3292: has variable name "len": tree mask = NULL_TREE, len = NULL_TREE, bias = NULL_TREE;
Line 3349: has variable name "len": HOST_WIDE_INT start = 0, len = 0;

Since they are never used simultaneously, such issue is not recognized for now.
However, I want to add LEN_MASK_{LOAD,STORE} which will need these 2 variables, so fix naming in this path.

Change HOST_WIDE_INT start = 0, len = 0; into HOST_WIDE_INT start = 0, length = 0;

gcc/ChangeLog:

	* tree-ssa-sccvn.cc (vn_reference_lookup_3): Change name "len" into
	"length".