[turbofan] Use NumAllocatableRegisters() instead of kMaxNumAllocatableRegisters.

Use NumAllocatableRegisters() instead of kMaxNumAllocatableRegisters in
ArchDefaultRegisterConfiguration::ArchDefaultRegisterConfiguration()
otherwise the compiler DCHECKS if triggered during snapshot builds on
Arm where only 16 DoubleRegisters are avaiable instead of 32.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1230473006

Cr-Commit-Position: refs/heads/master@{#29548}

src/compiler/register-configuration.cc

@@ -25,7 +25,7 @@ class ArchDefaultRegisterConfiguration : public RegisterConfiguration {
                               1,
                               1,
 #else
-                              DoubleRegister::kMaxNumAllocatableRegisters,
+                              DoubleRegister::NumAllocatableRegisters(),
                               DoubleRegister::NumAllocatableAliasedRegisters(),
 #endif
                               general_register_name_table_,
@@ -35,7 +35,9 @@ class ArchDefaultRegisterConfiguration : public RegisterConfiguration {
     for (int i = 0; i < Register::kMaxNumAllocatableRegisters; ++i) {
       general_register_name_table_[i] = Register::AllocationIndexToString(i);
     }
-    for (int i = 0; i < DoubleRegister::kMaxNumAllocatableRegisters; ++i) {
+    DCHECK_GE(DoubleRegister::kMaxNumAllocatableRegisters,
+              DoubleRegister::NumAllocatableRegisters());
+    for (int i = 0; i < DoubleRegister::NumAllocatableRegisters(); ++i) {
       double_register_name_table_[i] =
           DoubleRegister::AllocationIndexToString(i);
     }