Remove the return_reg instruction.

RISC architectures that take a return address in a register can use a
special-purpose `link` return value to do so.

filetests/isa/riscv/abi.cton

@@ -4,7 +4,7 @@ isa riscv
 
 ; regex: V=v\d+
 
-function f(i32) {
+function f() {
     sig0 = signature(i32) -> i32
     ; check: sig0 = signature(i32 [%x10]) -> i32 [%x10]
 
@@ -27,6 +27,6 @@ function f(i32) {
     sig5 = signature(i64x4)
     ; check: sig5 = signature(i32 [%x10], i32 [%x11], i32 [%x12], i32 [%x13], i32 [%x14], i32 [%x15], i32 [%x16], i32 [%x17])
 
-ebb0(v0: i32):
-    return_reg v0
+ebb0:
+    return
 }

filetests/isa/riscv/encoding.cton

@@ -15,7 +15,7 @@ ebb0(v1: i32, v2: i32):
     ; check: [R#10c]
     ; sameln: $v12 = imul
 
-    return_reg v1
+    return
     ; check: [Iret#19]
-    ; sameln: return_reg
+    ; sameln: return
 }

filetests/regalloc/basic.cton

@@ -8,5 +8,5 @@ ebb0(v1: i32, v2: i32):
     v3 = iadd v1, v2
 ; check: [R#0c,%x0]
 ; sameln: iadd
-    return_reg v3
+    return
 }