[RISCV] Add CSR instruction aliases (imm. operands)

[luismarques]

Summary

This patch adds CSR instructions aliases for the cases where the instruction takes an immediate operand but the alias doesn't have the i suffix. This is necessary for GCC compatibility.

[asb]

Looks good to me. I double-checked that gas doesn't do this same conversion for fsflags and fsrm. It doesn't, so this seems complete.

Thanks!

lib/Target/RISCV/RISCVInstrInfo.td

 def : InstAlias<"csrw $csr, $rs", (CSRRW      X0, csr_sysreg:$csr, GPR:$rs)>;
 def : InstAlias<"csrs $csr, $rs", (CSRRS      X0, csr_sysreg:$csr, GPR:$rs)>;
 def : InstAlias<"csrc $csr, $rs", (CSRRC      X0, csr_sysreg:$csr, GPR:$rs)>;
 
 def : InstAlias<"csrwi $csr, $imm", (CSRRWI X0, csr_sysreg:$csr, uimm5:$imm)>;
 def : InstAlias<"csrsi $csr, $imm", (CSRRSI X0, csr_sysreg:$csr, uimm5:$imm)>;
 def : InstAlias<"csrci $csr, $imm", (CSRRCI X0, csr_sysreg:$csr, uimm5:$imm)>;
 
+let EmitPriority = 0 in {
+def : InstAlias<"csrw $csr, $imm", (CSRRWI X0, csr_sysreg:$csr, uimm5:$imm)>;
+def : InstAlias<"csrs $csr, $imm", (CSRRSI X0, csr_sysreg:$csr, uimm5:$imm)>;
+def : InstAlias<"csrc $csr, $imm", (CSRRCI X0, csr_sysreg:$csr, uimm5:$imm)>;
+
+def : InstAlias<"csrrw $rd, $csr, $imm", (CSRRWI GPR:$rd, csr_sysreg:$csr, uimm5:$imm)>;
+def : InstAlias<"csrrs $rd, $csr, $imm", (CSRRSI GPR:$rd, csr_sysreg:$csr, uimm5:$imm)>;
+def : InstAlias<"csrrc $rd, $csr, $imm", (CSRRCI GPR:$rd, csr_sysreg:$csr, uimm5:$imm)>;
+}
+
 def : InstAlias<"sfence.vma",     (SFENCE_VMA      X0, X0)>;
 def : InstAlias<"sfence.vma $rs", (SFENCE_VMA GPR:$rs, X0)>;
 
 let EmitPriority = 0 in {
 def : InstAlias<"add $rd, $rs1, $imm12",
                 (ADDI  GPR:$rd, GPR:$rs1, simm12:$imm12)>;
 def : InstAlias<"and $rd, $rs1, $imm12",
                 (ANDI  GPR:$rd, GPR:$rs1, simm12:$imm12)>;

test/MC/RISCV/rvi-aliases-valid.s

 csrwi 0x150, 0xf
 # CHECK-S-OBJ-NOALIAS: csrrsi zero, 4095, 16
 # CHECK-S-OBJ: csrsi 4095, 16
 csrsi 0xfff, 0x10
 # CHECK-S-OBJ-NOALIAS: csrrci zero, sscratch, 17
 # CHECK-S-OBJ: csrci sscratch, 17
 csrci 0x140, 0x11
 
+# CHECK-S-OBJ-NOALIAS: csrrwi zero, 336, 7
+# CHECK-S-OBJ: csrwi 336, 7
+csrw 0x150, 7
+# CHECK-S-OBJ-NOALIAS: csrrsi zero, 336, 7
+# CHECK-S-OBJ: csrsi 336, 7
+csrs 0x150, 7
+# CHECK-S-OBJ-NOALIAS: csrrci zero, 336, 7
+# CHECK-S-OBJ: csrci 336, 7
+csrc 0x150, 7
+
+# CHECK-S-OBJ-NOALIAS: csrrwi t0, 336, 15
+# CHECK-S-OBJ: csrrwi t0, 336, 15
+csrrw t0, 0x150, 0xf
+# CHECK-S-OBJ-NOALIAS: csrrsi t0, 4095, 16
+# CHECK-S-OBJ: csrrsi t0, 4095, 16
+csrrs t0, 0xfff, 0x10
+# CHECK-S-OBJ-NOALIAS: csrrci t0, sscratch, 17
+# CHECK-S-OBJ: csrrci t0, sscratch, 17
+csrrc t0, 0x140, 0x11
+
 # CHECK-S-OBJ-NOALIAS: sfence.vma zero, zero
 # CHECK-S-OBJ: sfence.vma
 sfence.vma
 # CHECK-S-OBJ-NOALIAS: sfence.vma a0, zero
 # CHECK-S-OBJ: sfence.vma a0
 sfence.vma a0
 
 # The following aliases are accepted as input but the canonical form