Add a pressure_tracking flag to register banks.

This makes it possible to define register banks that opt out of register
pressure tracking. This will be used to define banks for special-purpose
registers like the CPU flags.

The pressure tracker does not need to use resources for a top-level
register class in a non-tracked bank. The constant MAX_TOPRCS is renamed
to MAX_TRACKED_TOPRCS to indicate that there may be top-level register
classes with higher numbers, but they won't require pressure tracking.

We won't be tracking register pressure for CPU flags since only one
value is allowed to be live at a time.

lib/cretonne/src/isa/registers.rs

@@ -27,10 +27,10 @@ pub type RegUnitMask = [u32; 3];
 /// This type should be coordinated with meta/cdsl/isa.py.
 pub type RegClassMask = u32;
 
-/// Guaranteed maximum number of top-level register classes in any ISA.
+/// Guaranteed maximum number of top-level register classes with pressure tracking in any ISA.
 ///
 /// This can be increased, but should be coordinated with meta/cdsl/isa.py.
-pub const MAX_TOPRCS: usize = 4;
+pub const MAX_TRACKED_TOPRCS: usize = 4;
 
 /// The register units in a target ISA are divided into disjoint register banks. Each bank covers a
 /// contiguous range of register units.
@@ -63,6 +63,9 @@ pub struct RegBank {
     /// The top-level register classes in a bank are guaranteed to be numbered sequentially from
     /// `first_toprc`, and all top-level register classes across banks come before any sub-classes.
     pub num_toprcs: usize,
+
+    /// Is register pressure tracking enabled for this bank?
+    pub pressure_tracking: bool,
 }
 
 impl RegBank {