Stores with invariant address unnecessarily widened with EVL tail folding

[lukel97]

https://godbolt.org/z/9a6b9oe5c

Given a loop that has a store to an invariant address:

define void @f(ptr %p, ptr %q, i32 %n) {
entry:
  br label %loop

loop:
  %iv = phi i32 [0, %entry], [%iv.next, %loop]
  %gep = getelementptr i32, ptr %p, i32 %iv
  %x = load i32, ptr %gep
  %y = add i32 %x, 1
  store i32 %y, ptr %gep
  store i32 %y, ptr %q ; address invariant
  %iv.next = add i32 %iv, 1
  %done = icmp eq i32 %iv.next, %n
  br i1 %done, label %exit, label %loop

exit:
  ret void
}

Typically we extract the corresponding element and emit a scalar store:

  %wide.load = load <vscale x 4 x i32>, ptr %14, align 4
  %15 = add <vscale x 4 x i32> %wide.load, splat (i32 1)
  %19 = extractelement <vscale x 4 x i32> %15, i32 %18
  store i32 %19, ptr %q, align 4

However with EVL tail folding we fail to do this and instead emit a scatter:

  %vp.op.load = call <vscale x 4 x i32> @llvm.vp.load.nxv4i32.p0(ptr align 4 %13, <vscale x 4 x i1> splat (i1 true), i32 %11)
  %14 = add <vscale x 4 x i32> %vp.op.load, splat (i32 1)
  call void @llvm.vp.store.nxv4i32.p0(<vscale x 4 x i32> %14, ptr align 4 %13, <vscale x 4 x i1> splat (i1 true), i32 %11)
  call void @llvm.vp.scatter.nxv4i32.nxv4p0(<vscale x 4 x i32> %14, <vscale x 4 x ptr> align 4 %broadcast.splat, <vscale x 4 x i1> splat (i1 true), i32 %11)