x64: port select to ISLE (#3682)

* x64: port `select` using an FP comparison to ISLE

This change includes quite a few interlocking parts, required mainly by
the current x64 conventions in ISLE:
 - it adds a way to emit a `cmove` with multiple OR-ing conditions;
   because x64 ISLE cannot currently safely emit a comparison followed
   by several jumps, this adds `MachInst::CmoveOr` and
   `MachInst::XmmCmoveOr` macro instructions. Unfortunately, these macro
   instructions hide the multi-instruction sequence in `lower.isle`
 - to properly keep track of what instructions consume and produce
   flags, @cfallin added a way to pass around variants of
   `ConsumesFlags` and `ProducesFlags`--these changes affect all
   backends
 - then, to lower the `fcmp + select` CLIF, this change adds several
   `cmove*_from_values` helpers that perform all of the awkward
   conversions between `Value`, `ValueReg`, `Reg`, and `Gpr/Xmm`; one
   upside is that now these lowerings have much-improved documentation
   explaining why the various `FloatCC` and `CC` choices are made the
   the way they are.

Co-authored-by: Chris Fallin <chris@cfallin.org>

cranelift/isle/isle/src/codegen.rs

@@ -460,7 +460,7 @@ impl<'a> Codegen<'a> {
                 };
                 let valuename = self.value_binder(&value, /* is_ref = */ true, ty);
                 let fieldname = &self.typeenv.syms[field.name.index()];
-                self.define_val(&value, ctx, /* is_ref = */ false, field.ty);
+                self.define_val(&value, ctx, /* is_ref = */ true, field.ty);
                 format!("{}: {}", fieldname, valuename)
             })
             .collect::<Vec<_>>()