Break out differently purposed tests from abi.cton.

- abi.cton is for testing the actual RISC-V ABI.
- legalize-abi.cton is for testing the legalizer around ABI boundaries.
- parse-encoding.cton is for testing the parser's handling of RISC-V
  encoding and register annotations.

filetests/isa/riscv/parse-encoding.cton

@@ -0,0 +1,36 @@
+; Test the parser's support for encoding annotations.
+test legalizer
+isa riscv
+
+function parse_encoding(i32 [%x5]) -> i32 [%x10] {
+    ; check: function parse_encoding(i32 [%x5]) -> i32 [%x10] {
+
+    sig0 = signature(i32 [%x10]) -> i32 [%x10]
+    ; check: sig0 = signature(i32 [%x10]) -> i32 [%x10]
+
+    sig1 = signature(i32 [%x10], i32 [%x11]) -> b1 [%x10]
+    ; check: sig1 = signature(i32 [%x10], i32 [%x11]) -> b1 [%x10]
+
+    sig2 = signature(f32 [%f10], i32 [%x12], i32 [%x13]) -> f64 [%f10]
+    ; check: sig2 = signature(f32 [%f10], i32 [%x12], i32 [%x13]) -> f64 [%f10]
+
+    ; Arguments on stack where not necessary
+    sig3 = signature(f64 [%f10], i32 [0], i32 [4]) -> f64 [%f10]
+    ; check: sig3 = signature(f64 [%f10], i32 [0], i32 [4]) -> f64 [%f10]
+
+    ; Stack argument before register argument
+    sig4 = signature(f32 [72], i32 [%x10])
+    ; check: sig4 = signature(f32 [72], i32 [%x10])
+
+    ; Return value on stack
+    sig5 = signature() -> f32 [0]
+    ; check: sig5 = signature() -> f32 [0]
+
+    ; function + signature
+    fn15 = function bar(i32 [%x10]) -> b1 [%x10]
+    ; check: sig6 = signature(i32 [%x10]) -> b1 [%x10]
+    ; nextln: fn0 = sig6 bar
+
+ebb0(v0: i32):
+    return v0
+}