wasm->CLIF translation: consistently bitcast V128 values that are block formal parameters.

In the current translation of wasm (128-bit) SIMD into CLIF, we work around differences in the
type system models of wasm vs CLIF by inserting `bitcast` (a no-op cast) CLIF instructions before
more or less every use of a SIMD value.  Unfortunately this was not being done consistently and
even small examples with a single if-then-else diamond that produces a SIMD value, could cause a
verification failure downstream.  In this case, the jump out of the "else" block needed a
bitcast, but didn't have one.

This patch wraps creation of CLIF jumps and conditional branches up into three functions,
`canonicalise_then_jump` and `canonicalise_then_br{z,nz}`, and uses them consistently.  They
first cast the relevant block formal parameters, then generate the relevant kind of branch/jump.
Hence, provided they are also used consistently in future to generate branches/jumps in this
file, we are protected against such failures.

The patch also adds a large(ish) comment at the top explaining this in more detail.

cranelift/wasmtests/pr2303.wat

@@ -0,0 +1,15 @@
+(module
+    (memory (export "mem") 1 1)
+    (func (export "runif") (param $cond i32)
+      i32.const 48
+      (v128.load (i32.const 0))
+      (v128.load (i32.const 16))
+      (if (param v128) (param v128) (result v128 v128)
+          (local.get $cond)
+          (then i64x2.add
+                (v128.load (i32.const 32)))
+          (else i32x4.sub
+                (v128.load (i32.const 0))))
+      i16x8.mul
+      v128.store)
+)