[InstCombine] Handle scalable splats of constants in getMinimumFPType

We previously handled ConstantExpr scalable splats in 5d92979, but only fpexts.

ConstantExpr fpexts have since been removed, and simultaneously we didn't handle splats of constants that weren't extended.

This updates it to remove the fpext check and instead see if we can shrink the result of getSplatValue.

Note that the test case doesn't get completely folded away due to #132922

Author: lukel97

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -1685,11 +1685,12 @@ static Type *getMinimumFPType(Value *V, bool PreferBFloat) {
       return T;
 
   // We can only correctly find a minimum type for a scalable vector when it is
-  // a splat. For splats of constant values the fpext is wrapped up as a
-  // ConstantExpr.
-  if (auto *FPCExt = dyn_cast<ConstantExpr>(V))
-    if (FPCExt->getOpcode() == Instruction::FPExt)
-      return FPCExt->getOperand(0)->getType();
+  // a splat.
+  if (auto *FPCE = dyn_cast<ConstantExpr>(V))
+    if (isa<ScalableVectorType>(V->getType()))
+      if (auto *Splat = dyn_cast<ConstantFP>(FPCE->getSplatValue()))
+        if (Type *T = shrinkFPConstant(Splat, PreferBFloat))
+          return T;
 
   // Try to shrink a vector of FP constants. This returns nullptr on scalable
   // vectors

llvm/test/Transforms/InstCombine/scalable-const-fp-splat.ll

@@ -17,9 +17,8 @@ define <vscale x 2 x float> @shrink_splat_scalable_extend(<vscale x 2 x float> %
 define <vscale x 2 x float> @shrink_splat_scalable_extend_rhs_constexpr(<vscale x 2 x float> %a) {
 ; CHECK-LABEL: define <vscale x 2 x float> @shrink_splat_scalable_extend_rhs_constexpr(
 ; CHECK-SAME: <vscale x 2 x float> [[A:%.*]]) {
-; CHECK-NEXT:    [[TMP1:%.*]] = fpext <vscale x 2 x float> [[A]] to <vscale x 2 x double>
-; CHECK-NEXT:    [[TMP2:%.*]] = fadd <vscale x 2 x double> [[TMP1]], splat (double -1.000000e+00)
-; CHECK-NEXT:    [[TMP3:%.*]] = fptrunc <vscale x 2 x double> [[TMP2]] to <vscale x 2 x float>
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc <vscale x 2 x double> splat (double -1.000000e+00) to <vscale x 2 x float>
+; CHECK-NEXT:    [[TMP3:%.*]] = fadd <vscale x 2 x float> [[A]], [[TMP1]]
 ; CHECK-NEXT:    ret <vscale x 2 x float> [[TMP3]]
 ;
   %2 = fpext <vscale x 2 x float> %a to <vscale x 2 x double>