PPC/s390: [arm] Reduce confusion between max and actual double register counts

Port 965916c7

Original Commit Message:

    The actual number of double regs available on arm depends on the
    current CPU. In some cases, we care about the max count of double regs
    (e.g.  around Save/RestoreFPRegs which always reserves spaces for the
    max); in others, we care about the actual count.

    Use DwVfpRegister::kNumRegisters to fetch the max (= 32).
    Use DwVfpRegister::SupportedRegisterCount() for the actual count (16
    or 32).

R=jgruber@chromium.org, joransiu@ca.ibm.com, junyan@redhat.com, midawson@redhat.com
BUG=
LOG=N

Change-Id: I55ef4a14b4a7676d9773f57ea66149a069c70448
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2502810Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#70807}

src/codegen/ppc/register-ppc.h

@@ -228,7 +228,11 @@ class DoubleRegister : public RegisterBase<DoubleRegister, kDoubleAfterLast> {
   // d14: 0.0
   // d15: scratch register.
   static constexpr int kSizeInBytes = 8;
-  inline static int NumRegisters();
+
+  // This function differs from kNumRegisters by returning the number of double
+  // registers supported by the current CPU, while kNumRegisters always returns
+  // 32.
+  inline static int SupportedRegisterCount();
 
  private:
   friend class RegisterBase;

src/codegen/s390/register-s390.h

@@ -186,7 +186,11 @@ class DoubleRegister : public RegisterBase<DoubleRegister, kDoubleAfterLast> {
   // d14: 0.0
   // d15: scratch register.
   static constexpr int kSizeInBytes = 8;
-  inline static int NumRegisters();
+
+  // This function differs from kNumRegisters by returning the number of double
+  // registers supported by the current CPU, while kNumRegisters always returns
+  // 32.
+  inline static int SupportedRegisterCount();
 
  private:
   friend class RegisterBase;