[APInt] Stop using soft-deprecated constructors and methods in llvm. NFC.

Stop using APInt constructors and methods that were soft-deprecated in
D109483. This fixes all the uses I found in llvm, except for the APInt
unit tests which should still test the deprecated methods.

Differential Revision: https://reviews.llvm.org/D110807
diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index 65cbbf2..c88dc96 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -4820,7 +4820,7 @@
   Register InH = MRI.createGenericVirtualRegister(HalfTy);
   MIRBuilder.buildUnmerge({InL, InH}, MI.getOperand(1));
 
-  if (Amt.isNullValue()) {
+  if (Amt.isZero()) {
     MIRBuilder.buildMerge(MI.getOperand(0), {InL, InH});
     MI.eraseFromParent();
     return Legalized;