[NFC][TableGen] Refactor IntrinsicEmitter code

- Use formatv() and raw string literals to simplify emission code.
- Use range based for loops and structured bindings to simplify loops.
- Use const Pointers to Records.
- Rename `ComputeFixedEncoding` to `ComputeTypeSignature` to reflect
  what the function actually does, cnd change it to return a vector.
- Use reverse() and for_each() for loop to pack 8 nibbles into 32-bits.
- Rename some variables to follow LLVM coding standards.

llvm/test/TableGen/intrinsic-attrs.td

@@ -60,7 +60,7 @@ def int_deref_ptr_ret : Intrinsic<[llvm_ptr_ty], [], [Dereferenceable<RetIndex,
 // CHECK-NEXT: default: llvm_unreachable("Invalid attribute set number");
 // CHECK-NEXT: case 0:
 // CHECK-NEXT:     return AttributeSet::get(C, {
-// CHECK-NEXT: Attribute::get(C, Attribute::Dereferenceable, 16),
+// CHECK-NEXT:       Attribute::get(C, Attribute::Dereferenceable, 16),
 // CHECK-NEXT: });
 // CHECK-NEXT: }
 // CHECK-NEXT: }