Fix value label ranges resolution (#1572)

There was a bug how value labels were resolved, which caused some DWARF expressions not be transformed, e.g. those are in the registers.

*    Implements FIXME in expression.rs
*    Move TargetIsa from CompiledExpression structure
*    Fix expression format for GDB
*    Add tests for parsing
*    Proper logic in ValueLabelRangesBuilder::process_label
*    Tests for ValueLabelRangesBuilder
*    Refactor build_with_locals to return Iterator instead of Vec<_>
*    Misc comments and magical numbers

cranelift/codegen/src/value_label.rs

@@ -18,9 +18,9 @@ use serde::{Deserialize, Serialize};
 pub struct ValueLocRange {
     /// The ValueLoc containing a ValueLabel during this range.
     pub loc: ValueLoc,
-    /// The start of the range.
+    /// The start of the range. It is an offset in the generated code.
     pub start: u32,
-    /// The end of the range.
+    /// The end of the range. It is an offset in the generated code.
     pub end: u32,
 }