[InstCombine] Teach getDemandedBitsLHSMask to handle constant splat vectors

[craig.topper]

Summary

This replaces a ConstantInt dyn_cast with m_APInt

[spatel]

LGTM.

lib/Transforms/InstCombine/InstCombineCompares.cpp

3939–3940

Could squash the unnecessary local variable into the return statement to eliminate the lowercase variable violation and remove the braces.

lib/Transforms/InstCombine/InstCombineCompares.cpp

 /// When performing a comparison against a constant, it is possible that not all
 /// the bits in the LHS are demanded. This helper method computes the mask that
 /// IS demanded.
 static APInt getDemandedBitsLHSMask(ICmpInst &I, unsigned BitWidth,
                                     bool isSignCheck) {
   if (isSignCheck)
     return APInt::getSignMask(BitWidth);
 
-  ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand(1));
-  if (!CI) return APInt::getAllOnesValue(BitWidth);
-  const APInt &RHS = CI->getValue();
+  const APInt *RHS;
+  if (!match(I.getOperand(1), m_APInt(RHS)))
+    return APInt::getAllOnesValue(BitWidth);
 
   switch (I.getPredicate()) {
   // For a UGT comparison, we don't care about any bits that
   // correspond to the trailing ones of the comparand.  The value of these
   // bits doesn't impact the outcome of the comparison, because any value
   // greater than the RHS must differ in a bit higher than these due to carry.
   case ICmpInst::ICMP_UGT: {
-    unsigned trailingOnes = RHS.countTrailingOnes();
+    unsigned trailingOnes = RHS->countTrailingOnes();
     return APInt::getBitsSetFrom(BitWidth, trailingOnes);
   }
 
   // Similarly, for a ULT comparison, we don't care about the trailing zeros.
   // Any value less than the RHS must differ in a higher bit because of carries.
   case ICmpInst::ICMP_ULT: {
-    unsigned trailingZeros = RHS.countTrailingZeros();
+    unsigned trailingZeros = RHS->countTrailingZeros();
     return APInt::getBitsSetFrom(BitWidth, trailingZeros);
   }
 
   default:
     return APInt::getAllOnesValue(BitWidth);
   }
 }
 

test/Transforms/InstCombine/icmp.ll

 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne <2 x i32> [[AND]], zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[CMP]]
 ;
   %and = and <2 x i32> %x, <i32 96, i32 96>
   %cmp = icmp ugt <2 x i32> %and, <i32 31, i32 31>
   ret <2 x i1> %cmp
 }
 
-; FIXME: Vector constant for the 'and' should use less bits.
 define <2 x i1> @test67vec2(<2 x i32> %x) {
 ; CHECK-LABEL: @test67vec2(
-; CHECK-NEXT:    [[AND:%.*]] = and <2 x i32> %x, <i32 127, i32 127>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt <2 x i32> [[AND]], <i32 31, i32 31>
+; CHECK-NEXT:    [[AND:%.*]] = and <2 x i32> [[X:%.*]], <i32 96, i32 96>
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ne <2 x i32> [[AND]], zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[CMP]]
 ;
   %and = and <2 x i32> %x, <i32 127, i32 127>
   %cmp = icmp ugt <2 x i32> %and, <i32 31, i32 31>
   ret <2 x i1> %cmp
 }
 
 define <2 x i1> @test67vecinverse(<2 x i32> %x) {