Refactor SIMD filetests to use a common naming convention

All filetests now should look like `simd-[instruction category]-[test type]`, where `[test type]` is something like `run` or `binemit`.

cranelift/filetests/filetests/isa/x86/simd-vconst-optimized-binemit.clif

@@ -0,0 +1,12 @@
+test binemit
+set enable_simd
+target x86_64
+
+; TODO move to vconst-compile.clif or vconst-binemit.clif
+
+function %test_vconst_optimizations() {
+block0:
+[-, %xmm4]  v0 = vconst.b8x16 0x00                                  ; bin: 66 0f ef e4
+[-, %xmm7]  v1 = vconst.b8x16 0xffffffffffffffffffffffffffffffff    ; bin: 66 0f 74 ff
+            return
+}