GlobalISel: Fix lowerSelect handling of boolean high bits

[arsenm]

Summary

This was making several invalid assumptions about the incoming
select. First, it was assuming the incoming condition was either s1 or
already sign extended, not accounting for different boolean high bits
behavior between scalar and vector conditions. We only had a vector
boolean due to the intermediate step vector select, which is now
avoided.

Second, it was assuming it can use the result vector type as a boolean
mask. These types don't have anything to do with other, and only makes
sense in the context of the expansion to bit operations. Since these
logically are part of the same lowering, do the complete expansion in
a single step.

The added select_v4s1_s1 test does fail to legalize, since it seems
AArch64's vector legalization support is pretty incomplete.

[arsenm]

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

7278–7279

Actually this whole promotion is wrong. There's no meaningful relationship between the bitwidth of the result type and the compare type. This also doesn't account for the original scalar being promoted

[arsenm]

This transform only makes sense to perform if the following bitwise expansion is done at the same time. I'm going to change this to do this legalize in one step

[arsenm]

Don't use broken intermediate vselect and ensure the boolean is appropriately extended

[paquette]

LGTM

[arsenm]

3f2cc7cc2b17ea6b7c09470ffc75b4691ce3ea95

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

LegalizerHelper::LegalizeResult LegalizerHelper::lowerSelect(MachineInstr &MI) {
   LLT MaskTy = MRI.getType(MaskReg);
   LLT Op1Ty = MRI.getType(Op1Reg);
   if (!DstTy.isVector())
     return UnableToLegalize;
 
   // Vector selects can have a scalar predicate. If so, splat into a vector and
   // finish for later legalization attempts to try again.
   if (MaskTy.isScalar()) {
+    // FIXME: We shouldn't be promoting the mask type here, or even the
+    // broadcast. The broadcast should probably be handled as MoreElements, not
+    // lower.
     Register MaskElt = MaskReg;
-    if (MaskTy.getSizeInBits() < DstTy.getScalarSizeInBits())
-      MaskElt = MIRBuilder.buildSExt(DstTy.getElementType(), MaskElt).getReg(0);
+    if (MaskTy.getSizeInBits() < DstTy.getScalarSizeInBits()) {
+      // FIXME: We have no way of knowing if this is FP without the original
+      // boolean's context.
+      MaskElt = MIRBuilder.buildBoolExt(DstTy.getElementType(),
+                                        MaskElt, false).getReg(0);
+    }
     // Generate a vector splat idiom to be pattern matched later.
     auto ShufSplat = MIRBuilder.buildShuffleSplat(DstTy, MaskElt);
     Observer.changingInstr(MI);
     MI.getOperand(1).setReg(ShufSplat.getReg(0));
     Observer.changedInstr(MI);
     return Legalized;
   }