Cranelift: disallow marking entry block 'cold'. (#4659)

This is a nonsensical constraint: the entry block must come first in the
compiled code's layout, so it cannot also be sunk to the end of the
function.

This PR modifies the CLIF verifier to disallow this situation entirely.
It also adds an assert during final block-order computation to catch the
problem (and avoid a silent miscompile) even if the verifier is
disabled.

Fixes #4656.

cranelift/filetests/filetests/verifier/cold_entry.clif

@@ -0,0 +1,6 @@
+test verifier
+
+function %entry_block_not_cold() {
+    block0 cold: ; error: entry block cannot be marked as cold
+        return
+}