[MLAS/CPU EP] Improve performance of Silu activation path within the QuickGelu CPU kernel (#26753)
### Description
The `Silu` activation is basically the same as `QuickGelu` but with the
scaling factor (`alpha`) as 1. In cusomer models containing `Silu`, the
graph optimizer suite correctly fuses the nodes into a QuickGelu with
alpha = 1. This optimizes the implementation of QuickGelu when alpha = 1
by avoiding the scaling and vectorizes the subsequent elementwise
multiplication.
**Tests:**
There are already tests for QuickGelu with alpha = 1 and there are no
new tests necessary
(https://github.com/microsoft/onnxruntime/blob/f98c756b45b81520c6e2a09c370575a013f02cce/onnxruntime/test/contrib_ops/activation_op_test.cc#L126)
**Performance improvements measured:**
Gives about 2.5% throughput boost for a customer model that has a lot of
Silu activations.
### Motivation and Context
Some low hanging fruit perf improvements that give instant easy perf
wins
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>