[AMDGPU] Remove constexpr from getNumUserSGPRForField/getMaxNumPreloadedSGPRs to appease older gcc builds

Older versions of gcc wouldn't accept the constexpr getNumUserSGPRForField (introduced in D159439 / 343be51) as it couldn't treat the llvm_unreachable call as constexpr

Author: RKSimon

llvm/lib/Target/AMDGPU/AMDGPUSubtarget.cpp

@@ -771,7 +771,7 @@ unsigned GCNSubtarget::getMaxNumSGPRs(const MachineFunction &MF) const {
                             getReservedNumSGPRs(MF));
 }
 
-static constexpr unsigned getMaxNumPreloadedSGPRs() {
+static unsigned getMaxNumPreloadedSGPRs() {
   using USI = GCNUserSGPRUsageInfo;
   // Max number of user SGPRs
   const unsigned MaxUserSGPRs =

llvm/lib/Target/AMDGPU/GCNSubtarget.h

@@ -1409,7 +1409,7 @@ class GCNUserSGPRUsageInfo {
   };
 
   // Returns the size in number of SGPRs for preload user SGPR field.
-  static constexpr unsigned getNumUserSGPRForField(UserSGPRID ID) {
+  static unsigned getNumUserSGPRForField(UserSGPRID ID) {
     switch (ID) {
     case ImplicitBufferPtrID:
       return 2;