tree-optimization/103641 - improve vect_synth_mult_by_constant

tree-optimization/103641 - improve vect_synth_mult_by_constant

The following happens to improve compile-time of the PR103641
testcase on aarch64 significantly.  I did not investigate the
effect on the generated code but at least in theory
choose_mult_variant should do a better job when we tell it
the actual mode we are going to use for the operations it
synthesizes.

2022-02-04  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103641
	* tree-vect-patterns.cc (vect_synth_mult_by_constant):
	Pass the vector mode to choose_mult_variant.