[liftoff] Minor fix to RegClass definition

It would be nice if unused {RegClass}es just equal {kNoReg} on all
platforms.
This CL adds the static asserts for that, and slightly modifies the
definition of the {RegClass} enum to fulfil that. Interestingly, this
only changes the values for the one configuration that is currently
unused :)

R=zhin@chromium.org

Change-Id: Ib1b2a663183ad34822f514d6168c65450c1d7f2e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1975753Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65527}

src/wasm/baseline/liftoff-register.h

@@ -22,15 +22,15 @@ static constexpr bool kNeedS128RegPair = !kSimpleFPAliasing;
 enum RegClass : uint8_t {
   kGpReg,
   kFpReg,
-  kGpRegPair,
-  kFpRegPair = kGpRegPair + kNeedI64RegPair,
+  kGpRegPair = kFpReg + 1 + (kNeedS128RegPair && !kNeedI64RegPair),
+  kFpRegPair = kFpReg + 1 + kNeedI64RegPair,
   kNoReg = kFpRegPair + kNeedS128RegPair,
   // +------------------+-------------------------------+
   // |                  |        kNeedI64RegPair        |
   // +------------------+---------------+---------------+
   // | kNeedS128RegPair |     true      |    false      |
   // +------------------+---------------+---------------+
-  // |             true | 0,1,2,3,4 (a) | 0,1,2,2,3     |
+  // |             true | 0,1,2,3,4 (a) | 0,1,3,2,3     |
   // |            false | 0,1,2,3,3 (b) | 0,1,2,2,2 (c) |
   // +------------------+---------------+---------------+
   // (a) arm
@@ -38,6 +38,11 @@ enum RegClass : uint8_t {
   // (c) x64, arm64
 };
 
+static_assert(kNeedI64RegPair == (kGpRegPair != kNoReg),
+              "kGpRegPair equals kNoReg if unused");
+static_assert(kNeedS128RegPair == (kFpRegPair != kNoReg),
+              "kFpRegPair equals kNoReg if unused");
+
 enum RegPairHalf : uint8_t { kLowWord = 0, kHighWord = 1 };
 
 static inline constexpr bool needs_reg_pair(ValueType type) {