[InstCombine] Handle scalable splats of constants in getMinimumFPType (#132960)

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

@@ -1684,12 +1684,12 @@ static Type *getMinimumFPType(Value *V, bool PreferBFloat) {
     if (Type *T = shrinkFPConstant(CFP, 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();
+  // Try to shrink scalable and fixed splat vectors.
+  if (auto *FPC = dyn_cast<Constant>(V))
+    if (isa<VectorType>(V->getType()))
+      if (auto *Splat = dyn_cast_or_null<ConstantFP>(FPC->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/fpextend.ll

@@ -448,3 +448,14 @@ define bfloat @bf16_frem(bfloat %x) {
   %t3 = fptrunc float %t2 to bfloat
   ret bfloat %t3
 }
+
+define <4 x float> @v4f32_fadd(<4 x float> %a) {
+; CHECK-LABEL: @v4f32_fadd(
+; CHECK-NEXT:    [[TMP1:%.*]] = fadd <4 x float> [[A:%.*]], splat (float -1.000000e+00)
+; CHECK-NEXT:    ret <4 x float> [[TMP1]]
+;
+  %2 = fpext <4 x float> %a to <4 x double>
+  %4 = fadd <4 x double> %2, splat (double -1.000000e+00)
+  %5 = fptrunc <4 x double> %4 to <4 x float>
+  ret <4 x float> %5
+}

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

@@ -13,3 +13,16 @@ define <vscale x 2 x float> @shrink_splat_scalable_extend(<vscale x 2 x float> %
   %5 = fptrunc <vscale x 2 x double> %4 to <vscale x 2 x float>
   ret <vscale x 2 x float> %5
 }
+
+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:%.*]] = 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>
+  %4 = fadd <vscale x 2 x double> %2, splat (double -1.000000e+00)
+  %5 = fptrunc <vscale x 2 x double> %4 to <vscale x 2 x float>
+  ret <vscale x 2 x float> %5
+}