Reapply [ConstantInt] Disable implicit truncation in ConstantInt::get() (#171456)

Reapply after additional fixes in #174426 and #174431.

-----

Disable implicit truncation in the ConstantInt constructor by default.
This means that it needs to be passed a signed/unsigned (depending on
the IsSigned flag) value matching the bit width.

The intention is to prevent the recurring bug where people write
something like `ConstantInt::get(Ty, -1)`, and this "works" until `Ty`
is larger than 64-bit and then the value is incorrect due to missing
type extension.

This is the continuation of
#112670, which originally
allowed implicit truncation in this constructor to reduce initial scope
of the change.

Author: nikic

llvm/include/llvm/IR/Constants.h

@@ -113,33 +113,30 @@ class ConstantInt final : public ConstantData {
   /// If Ty is a vector type, return a Constant with a splat of the given
   /// value. Otherwise return a ConstantInt for the given value.
   /// \param ImplicitTrunc Whether to allow implicit truncation of the value.
-  // TODO: Make ImplicitTrunc default to false.
   LLVM_ABI static Constant *get(Type *Ty, uint64_t V, bool IsSigned = false,
-                                bool ImplicitTrunc = true);
+                                bool ImplicitTrunc = false);
 
   /// Return a ConstantInt with the specified integer value for the specified
   /// type. If the type is wider than 64 bits, the value will be zero-extended
   /// to fit the type, unless IsSigned is true, in which case the value will
   /// be interpreted as a 64-bit signed integer and sign-extended to fit
   /// the type.
   /// \param ImplicitTrunc Whether to allow implicit truncation of the value.
-  // TODO: Make ImplicitTrunc default to false.
   LLVM_ABI static ConstantInt *get(IntegerType *Ty, uint64_t V,
                                    bool IsSigned = false,
-                                   bool ImplicitTrunc = true);
+                                   bool ImplicitTrunc = false);
 
   /// Return a ConstantInt with the specified value for the specified type. The
   /// value V will be canonicalized to an unsigned APInt. Accessing it with
   /// either getSExtValue() or getZExtValue() will yield a correctly sized and
   /// signed value for the type Ty.
   /// Get a ConstantInt for a specific signed value.
   /// \param ImplicitTrunc Whether to allow implicit truncation of the value.
-  // TODO: Make ImplicitTrunc default to false.
   static ConstantInt *getSigned(IntegerType *Ty, int64_t V,
-                                bool ImplicitTrunc = true) {
+                                bool ImplicitTrunc = false) {
     return get(Ty, V, /*IsSigned=*/true, ImplicitTrunc);
   }
-  static Constant *getSigned(Type *Ty, int64_t V, bool ImplicitTrunc = true) {
+  static Constant *getSigned(Type *Ty, int64_t V, bool ImplicitTrunc = false) {
     return get(Ty, V, /*IsSigned=*/true, ImplicitTrunc);
   }