Add MC support of RISCV Zcd Extension

[VincentWu]

Summary

This patch add the instructions of Zcd extension.

Zcd is a subset of C Ext which include the double-precision floating-point instructions (c.fld, c.fldsp, c.fsd, c.fsdsp).

[craig.topper]

Need to add Zcd command lines to compress-rv32d.s

clang/test/Preprocessor/riscv-target-features.c

41–46

Need to update this too.

llvm/lib/Target/RISCV/RISCVInstrInfoC.td

881

CompressPats need their predicates updated too. They are used by the assembler so they are MC layer.

llvm/test/MC/RISCV/rv32dc-valid.s

1

Need to add Zcd command lines

[craig.topper]

llvm/lib/Target/RISCV/RISCV.td

318

This predicate isn't used

[VincentWu]

address comments

[VincentWu]

fix testcase

[craig.topper]

Need to add Zcd command lines to compress-rv32d.s

[VincentWu]

address comment

[craig.topper]

LGTM

[VincentWu]

rebase