Add x86 shuffle implemention

[abrown]

This uses work pending review in #943 and adds some encodings for boolean comparisons in order to allow more complete testing of the shuffle implementation.

[bnjbvr]

I only looked at the first commit. I wanted to ask about the second one: why do we need all these new encodings for boolean types?

Looking great otherwise, this is on the right track. (Bonus points if you can split the Immediate -> Constant renaming from the first patch, to make it slightly easier to review; don't worry if this is too much hassle.)

[abrown]

I added the boolean encodings solely in order to write a binary shuffle test: https://github.com/CraneStation/cranelift/blob/27d809a444c6030f4f929153c9e9d5f7456c98a2/filetests/isa/x86/shuffle-run.clif#L47-L58. I can move that to a separate PR but I guess I felt at the time I wrote this that because the binary encodings exactly matched the pre-existing integer encodings the change was relatively safe.

[bnjbvr]

Looks great, thank you very much! Thanks for explaining the boolean encodings, it makes sense.