Encode iconst.i32 for RISC-V.

For large constants with the low 12 bits clear, we already have the
"lui" encoding. Add "addi %x0" encodings for signed 12-bit constants.

filetests/isa/riscv/binary32.cton

@@ -77,6 +77,9 @@ ebb0(v9999: i32):
     ; lui
     [-,%x7]     v140 = iconst.i32 0x12345000            ; bin: 123453b7
     [-,%x16]    v141 = iconst.i32 0xffffffff_fedcb000   ; bin: fedcb837
+    ; addi
+    [-,%x7]     v142 = iconst.i32 1000                  ; bin: 3e800393
+    [-,%x16]    v143 = iconst.i32 -905                  ; bin: c7700813
 
     ; Copies alias to iadd_imm.
     [-,%x7]     v150 = copy v1                          ; bin: 00050393

filetests/isa/riscv/verify-encoding.cton

@@ -6,7 +6,7 @@ function %RV32I(i32 link [%x1]) -> i32 link [%x1] {
 
 ebb0(v9999: i32):
     ; iconst.i32 needs legalizing, so it should throw a
-    [R#0,-]         v1 = iconst.i32 1 ; error: Instruction failed to re-encode
+    [R#0,-]         v1 = iconst.i32 0xf0f0f0f0f0 ; error: Instruction failed to re-encode
     return v9999
 }