Improve documentation of the filetest run command (#1645)

* Improve output display of RunCommand

The previous use of Debug for displaying `print` and `run` results was less than clear.

* Avoid checking the types of vectors during trampoline construction

Because DataValue only understands `V128` vectors, we avoid type-checking vector values when constructing the trampoline arguments.

* Improve the documentation of the filetest `run` command

Adds an up-to-date example of how to use the `run` and `print` directives and includes an actual use of the new directives in a SIMD arithmetic filetest.

cranelift/filetests/filetests/isa/x86/simd-arithmetic-run.clif

@@ -2,37 +2,21 @@ test run
 set enable_simd
 target x86_64 skylake
 
-function %iadd_i32x4() -> b1 {
-block0:
-    v0 = vconst.i32x4 [1 1 1 1]
-    v1 = vconst.i32x4 [1 2 3 4]
+function %iadd_i32x4(i32x4, i32x4) -> i32x4 {
+block0(v0:i32x4, v1:i32x4):
     v2 = iadd v0, v1
-
-    v3 = extractlane v2, 0
-    v4 = icmp_imm eq v3, 2
-
-    v5 = extractlane v2, 3
-    v6 = icmp_imm eq v5, 5
-    ; TODO replace extractlanes with vector comparison
-
-    v7 = band v4, v6
-    return v7
+    return v2
 }
-; run
+; run: %iadd_i32x4([1 1 1 1], [1 2 3 4]) == [2 3 4 5]
 
-function %iadd_i8x16_with_overflow() -> b1 {
+function %iadd_i8x16_with_overflow() -> i8x16 {
 block0:
     v0 = vconst.i8x16 [255 255 255 255 255 255 255 255 255 255 255 255 255 255 255 255]
     v1 = vconst.i8x16 [2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2]
     v2 = iadd v0, v1
-
-    v3 = extractlane v2, 0
-    v4 = icmp_imm eq v3, 1
-    ; TODO replace extractlane with vector comparison
-
-    return v4
+    return v2
 }
-; run
+; run: %iadd_i8x16_with_overflow() == [1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1]
 
 function %isub_i32x4_rex() -> b1 {
 block0: