[mlir] [VectorOps] Replace zero fma with mult for vector.contract

More efficient implementation of the multiply-reduce pair,
no need to add in a zero vector. Microbenchmarking on AVX2
yields the following difference in vector.contract speedup
(over strict-order scalar reduction).

SPEEDUP     SIMD-fma SIMD-mul
4x4	    1.45 	 2.00
8x8	    1.40 	 1.90
32x32    	5.32 	 5.80

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D82833