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[RISCV] DAG nodes and pseudo instructions for CSR access

Authored by sepavloff on Mar 18 2021, 8:07 AM.

Description

[RISCV] DAG nodes and pseudo instructions for CSR access

New custom DAG nodes were added to represent operations on CSR. These
nodes are lowered to corresponding pseudo instruction. Using the pseudo
instructions allows to specify different scheduling information for
operations on different system registers. It also make possible to
specify dependencies of instructions on specific system registers.

Differential Revision: https://reviews.llvm.org/D98936

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