vect: Complete implementation for MULT_EXPR vector lowering.HEADtrunkmaster

Use sequences of shifts and add/sub if the hardware does not have support for
vector multiplication. In a previous patch, bare bones vector lowering had been
implemented which only worked when the constant value was a power of 2.

In this patch, few more cases have been added, i.e., if a constant is a uniform
vector but not a power of 2 then use the choose_mult_variant, with max cost
estimate as the cost of scalar multiplication operation times the number of
elements in the vector. This is similar to the logic while expanding MULT_EXPR
in expand pass or in the vector pattern recognition in tree-vect-patterns.cc.

2025-11-08  Avinash Jayakar  <avinashd@linux.ibm.com>

gcc/ChangeLog:
	PR tree-optimization/122065
	* tree-vect-generic.cc (target_supports_mult_synth_alg): Add helper to
	check mult synth.
	(expand_vector_mult): Optimize mult when const is uniform but not
	power of 2.

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