tree-optimization/116610 - wrong SLP induction bias for mask peeling

The following fixes a mistake when applying the bias for peeling via
masking to the inital value of SLP inductions.

This resolves gcc.target/aarch64/sve/peel_ind_1.c (a scan-assembler
only unfortunately) when forcing single-lane SLP for it.

	PR tree-optimization/116610
	* tree-vect-loop.cc (vectorizable_induction): Use MINUS_EXPR
	to apply a mask peeling adjustment.