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[RISCV] Optimize add-mul in the zba extension with SH*ADD

Authored by benshi001 on Jun 18 2021, 8:01 PM.

Description

[RISCV] Optimize add-mul in the zba extension with SH*ADD

This patch does the following optimization.

Rx + Ry * 18 => (SH1ADD (SH3ADD Rx, Rx), Ry)
Rx + Ry * 20 => (SH2ADD (SH2ADD Rx, Rx), Ry)
Rx + Ry * 24 => (SH3ADD (SH1ADD Rx, Rx), Ry)
Rx + Ry * 36 => (SH2ADD (SH3ADD Rx, Rx), Ry)
Rx + Ry * 40 => (SH3ADD (SH2ADD Rx, Rx), Ry)
Rx + Ry * 72 => (SH3ADD (SH3ADD Rx, Rx), Ry)

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D104588