[RISCV] Update matchSplatAsGather to use the index of extract_elt if it is in-bounds

This a follow up to llvm#117878 and allows the usage of vrgather if the
index we are accessing in VT is a constant and within bounds.

This patch replaces the previous behavior of bailing out if the length
of the search vector was greater than the vector of elements we are
searching for. Since matchSplatAsGather works on EXTRACT_VECTOR_ELT, and
we know the index where the element is being extracted from, we can use
safely use vrgather.

Author: mikhailramalho

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -3509,9 +3509,9 @@ static SDValue matchSplatAsGather(SDValue SplatVal, MVT VT, const SDLoc &DL,
     return SDValue();
 
   // Check that Index lies within VT
-  // TODO: Can we check if the Index is constant and known in-bounds?
-  if (!TypeSize::isKnownLE(Vec.getValueSizeInBits(), VT.getSizeInBits()))
-    return SDValue();
+  if (auto *CIdx = dyn_cast<ConstantSDNode>(Idx))
+    if (VT.getVectorElementCount().getKnownMinValue() <= CIdx->getZExtValue())
+      return SDValue();
 
   MVT ContainerVT = VT;
   if (VT.isFixedLengthVector())