[CodeGenTypes] Remove redundant declarations (NFC) (llvm#165972)

In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

This patch also removes redundant "const" from the in-class definitions.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/include/llvm/CodeGenTypes/LowLevelType.h

@@ -340,18 +340,18 @@ class LLT {
   ///   valid encodings, SizeInBits/SizeOfElement must be larger than 0.
   /// * Non-pointer scalar (isPointer == 0 && isVector == 0):
   ///   SizeInBits: 32;
-  static const constexpr BitFieldInfo ScalarSizeFieldInfo{32, 29};
+  static constexpr BitFieldInfo ScalarSizeFieldInfo{32, 29};
   /// * Pointer (isPointer == 1 && isVector == 0):
   ///   SizeInBits: 16;
   ///   AddressSpace: 24;
-  static const constexpr BitFieldInfo PointerSizeFieldInfo{16, 45};
-  static const constexpr BitFieldInfo PointerAddressSpaceFieldInfo{24, 21};
+  static constexpr BitFieldInfo PointerSizeFieldInfo{16, 45};
+  static constexpr BitFieldInfo PointerAddressSpaceFieldInfo{24, 21};
   /// * Vector-of-non-pointer (isPointer == 0 && isVector == 1):
   ///   NumElements: 16;
   ///   SizeOfElement: 32;
   ///   Scalable: 1;
-  static const constexpr BitFieldInfo VectorElementsFieldInfo{16, 5};
-  static const constexpr BitFieldInfo VectorScalableFieldInfo{1, 0};
+  static constexpr BitFieldInfo VectorElementsFieldInfo{16, 5};
+  static constexpr BitFieldInfo VectorScalableFieldInfo{1, 0};
   /// * Vector-of-pointer (isPointer == 1 && isVector == 1):
   ///   NumElements: 16;
   ///   SizeOfElement: 16;

llvm/lib/CodeGenTypes/LowLevelType.cpp

@@ -54,9 +54,3 @@ LLVM_DUMP_METHOD void LLT::dump() const {
   dbgs() << '\n';
 }
 #endif
-
-const constexpr LLT::BitFieldInfo LLT::ScalarSizeFieldInfo;
-const constexpr LLT::BitFieldInfo LLT::PointerSizeFieldInfo;
-const constexpr LLT::BitFieldInfo LLT::PointerAddressSpaceFieldInfo;
-const constexpr LLT::BitFieldInfo LLT::VectorElementsFieldInfo;
-const constexpr LLT::BitFieldInfo LLT::VectorScalableFieldInfo;