[ARM,MVE] Add the vmovlbq,vmovltq intrinsic family.

Summary:
These intrinsics take a vector of 2n elements, and return a vector of
n wider elements obtained by sign- or zero-extending every other
element of the input vector. They're represented in IR as a
shufflevector that extracts the odd or even elements of the input,
followed by a sext or zext.

Existing LLVM codegen already matches this pattern and generates the
VMOVLB instruction (which widens the even-index input lanes). But no
existing isel rule was generating VMOVLT, so I've added some. However,
the new rules currently only work in little-endian MVE, because the
pattern they expect from isel lowering includes a bitconvert which
doesn't have the right semantics in big-endian.

The output of one existing codegen test is improved by those new
rules.

This commit adds the unpredicated forms only.

Reviewers: dmgreen, miyuki, MarkMurrayARM, ostannard

Reviewed By: dmgreen

Subscribers: kristof.beyls, hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D74336
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 788f14b..b30a79a0 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -7056,6 +7056,17 @@
   }
 }
 
+static llvm::Value *VectorUnzip(CGBuilderTy &Builder, llvm::Value *V, bool Odd) {
+  // Make a shufflevector that extracts every other element of a vector (evens
+  // or odds, as desired).
+  SmallVector<uint32_t, 16> Indices;
+  unsigned InputElements = V->getType()->getVectorNumElements();
+  for (unsigned i = 0; i < InputElements; i += 2)
+    Indices.push_back(i + Odd);
+  return Builder.CreateShuffleVector(V, llvm::UndefValue::get(V->getType()),
+                                     Indices);
+}
+
 template<unsigned HighBit, unsigned OtherBits>
 static llvm::Value *ARMMVEConstantSplat(CGBuilderTy &Builder, llvm::Type *VT) {
   // MVE-specific helper function to make a vector splat of a constant such as