[Hexagon] Simplify Mul->Sub->Conv to Conv->Add when possible

[arangasa]

When one of the inputs to Mul and Sub are constant scalars, we have the following algebraic identity:
Let res[p,q,r,s] denote the result of Mul(a, c1)->Sub(c2)->Conv(W) pattern
res[p,q,r,s] = Conv(ac1 - c2, W)
= SUM{i=[0,c-1], j=[0,kh-1], k=[0,kw-1]}
{(a[p,i,r+j,s+k] * c1 - c2) * W[q,i,j,k]}
= SUM{i=[0,c-1], j=[0,kh-1], k=[0,kw-1]}
{a[p,i,r+j,s+k] * c1 * W[q,i,j,k]} - c2 * W[q,i,j,k]}
= Conv(a, Wc1) + Conv(0-c2, W)
Since Conv(0-c2,W) and W*c1 are constant terms, they could be computed during compile time.

[tvm-bot]

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• cc @ibsidorenko _{See #10317 for details}

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[arangasa]

@kparzysz-quic @quic-sanirudh

[quic-sanirudh]

LGTM, thanks.