[AArch64] add missing pattern for insert_subvector undef

The attached testcase started failing after the patch to define
isExtractSubvectorCheap with the following pattern mismatch:

ISEL: Starting pattern match
  Initial Opcode index to 85068
    Match failed at index 85076
    LLVM ERROR: Cannot select: t47: v8i16 = insert_subvector undef:v8i16, t43, Constant:i64<0>

The code generated from llvm/lib/Target/AArch64/AArch64InstrInfo.td

def : Pat<(insert_subvector undef, (v4i16 FPR64:$src), (i32 0)),
          (INSERT_SUBREG (v8i16 (IMPLICIT_DEF)), FPR64:$src, dsub)>;

is in ninja/lib/Target/AArch64/AArch64GenDAGISel.inc
At the location of the error it is:
/* 85076*/    OPC_CheckChild2Type, MVT::i32,

And it failed to match the type of operand 2.
Adding another def-pat for i64 fixes the failed def-pat error:

def : Pat<(insert_subvector undef, (v4i16 FPR64:$src), (i64 0)),
          (INSERT_SUBREG (v8i16 (IMPLICIT_DEF)), FPR64:$src, dsub)>;

llvm-svn: 326949