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/wasm/Cargo.toml

@@ -20,6 +20,7 @@ hashbrown = { version = "0.7", optional = true }
 itertools = "0.9.0"
 log = { version = "0.4.6", default-features = false }
 serde = { version = "1.0.94", features = ["derive"], optional = true }
+smallvec = "1.0.0"
 thiserror = "1.0.4"
 
 [dev-dependencies]