Do value-extensions at ABI boundaries only when ABI requires it.

There has been some confusion over the meaning of the "sign-extend"
(`sext`) and "zero-extend" (`uext`) attributes on parameters and return
values in signatures. According to the three implemented backends, these
attributes indicate that a value narrower than a full register should
always be extended in the way specified. However, they are much more
useful if they mean "extend in this way if the ABI requires extending":
only the ABI backend knows whether or not a particular ABI (e.g., x64
SysV vs. x64 Baldrdash) requires extensions, while only the frontend
(CLIF generator) knows whether or not a value is signed, so the two have
to work in concert.

This is the result of some very helpful discussion in #2354 (thanks to
@uweigand for raising the issue and @bjorn3 for helping to reason about
it).

This change respects the extension attributes in the above way, rather
than unconditionally extending, to avoid potential performance
degradation as we introduce more extension attributes on signatures.

cranelift/codegen/src/isa/x86/abi.rs

@@ -144,8 +144,13 @@ impl ArgAssigner for Args {
             return ValueConversion::VectorSplit.into();
         }
 
-        // Small integers are extended to the size of a pointer register.
-        if ty.is_int() && ty.bits() < u16::from(self.pointer_bits) {
+        // Small integers are extended to the size of a pointer register, but
+        // only in ABIs that require this. The Baldrdash (SpiderMonkey) ABI
+        // does, but our other supported ABIs on x86 do not.
+        if ty.is_int()
+            && ty.bits() < u16::from(self.pointer_bits)
+            && self.call_conv.extends_baldrdash()
+        {
             match arg.extension {
                 ArgumentExtension::None => {}
                 ArgumentExtension::Uext => return ValueConversion::Uext(self.pointer_type).into(),