[GISEL] G_SPLAT_VECTOR can take a splat that is smaller than the vector element

This is what SelectionDAG does. We'd like to reuse SelectionDAG patterns.

Author: michaelmaitland

llvm/docs/GlobalISel/GenericOpcode.rst

@@ -690,6 +690,10 @@ G_SPLAT_VECTOR
 
 Create a vector where all elements are the scalar from the source operand.
 
+The type of the operand must be equal to or smaller than the vector element
+type. If the operand is smaller than the vector element type, the scalar is
+implicitly truncated to the vector element type.
+
 Vector Reduction Operations
 ---------------------------
 

llvm/lib/CodeGen/MachineVerifier.cpp

@@ -1774,9 +1774,10 @@ void MachineVerifier::verifyPreISelGenericInstruction(const MachineInstr *MI) {
     if (!SrcTy.isScalar())
       report("Source type must be a scalar", MI);
 
-    if (DstTy.getScalarType() != SrcTy)
-      report("Element type of the destination must be the same type as the "
-             "source type",
+    if (TypeSize::isKnownGT(DstTy.getScalarType().getSizeInBits(),
+                            SrcTy.getSizeInBits()))
+      report("Element type of the destination must be the same size or smaller "
+             "than the source type",
              MI);
 
     break;

llvm/test/MachineVerifier/test_g_splat_vector.mir

@@ -22,6 +22,6 @@ body:             |
     ; CHECK: Source type must be a scalar
     %6:_(<vscale x 2 x s32>) = G_SPLAT_VECTOR %2
 
-    ; CHECK: Element type of the destination must be the same type as the source type
-    %7:_(<vscale x 2 x s64>) = G_SPLAT_VECTOR %0
+    ; CHECK: Element type of the destination must be the same size or smaller than the source type
+    %7:_(<vscale x 2 x s128>) = G_SPLAT_VECTOR %0
 ...