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[RISCV] Add RISCVISD::BR_CC similar to RISCVISD::SELECT_CC.

Authored by craig.topper on Mar 15 2021, 11:44 AM.

Description

[RISCV] Add RISCVISD::BR_CC similar to RISCVISD::SELECT_CC.

This allows me to introduce similar combines for branches as
we have recently added for SELECT_CC. Some of them are less
useful for standalone setccs and only help branch instructions.
By having a BR_CC node its easier to only affect branches.

I'm using CondCodeSDNode to make isel patterns easier to
write so we can refer to the codes by name. SELECT_CC uses a
constant instead.

I've translated the condition code just like SELECT_CC so
we need less patterns for the swapped conditions. This
includes special cases for X < 1 and X > -1 that get translated
to blez and bgez by using a 0 constant.

computeKnownBitsForTargetNode support for SELECT_CC is added
to allow MaskedValueIsZero to work for cases where the true
and false values of the SELECT_CC are setccs and the
result of the SELECT_CC is used by a BR_CC. This was needed
to avoid regressions in some of the overflow tests.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D98159