Implement IR intrinsics for gather prefetch.

Implement IR intrinsics for gather prefetch.

Summary:
Intrinsics and relative codegen has been implemented for the following
SVE instructions:

1. PRF<T> <prfop>, <Pg>, [<Xn|SP>, <Zm>.S, <mod>] -> 32-bit          scaled offset
2. PRF<T> <prfop>, <Pg>, [<Xn|SP>, <Zm>.D, <mod>] -> 32-bit unpacked scaled offset
3. PRF<T> <prfop>, <Pg>, [<Xn|SP>, <Zm>.D]        -> 64-bit          scaled offset
4. PRF<T> <prfop>, <Pg>, [<Zn>.S{, #<imm>}]       -> 32-bit element
5. PRF<T> <prfop>, <Pg>, [<Zn>.D{, #<imm>}]       -> 64-bit element

The instructions are associated the following intrinsics, respectively:

1. void @llvm.aarch64.sve.gather.prf<T>.scaled.<mod>.nx4vi32(
          i8* %base,
          <vscale x 4 x i32> %offset,
          <vscale x 4 x i1> %Pg,
          i32 %prfop)

2. void @llvm.aarch64.sve.gather.prf<T>.scaled.<mod>.nx2vi32(
          i8* %base,
          <vscale x 2 x i32> %offset,
          <vscale x 2 x i1> %Pg,
          i32 %prfop)

3. void @llvm.aarch64.sve.gather.prf<T>.scaled.nx2vi64(
          i8* %base,
          <vscale x 2 x i64> %offset,
          <vscale x 2 x i1> %Pg,
          i32 %prfop)

4. void @llvm.aarch64.sve.gather.prf<T>.nx4vi32(
          <vscale x 4 x i32> %bases,
          i64 %imm,
          <vscale x 4 x i1> %Pg,
          i32 %prfop)

5. void @llvm.aarch64.sve.gather.prf<T>.nx2vi64(
          <vscale x 2 x i64> %bases,
          i64 %imm,
          <vscale x 2 x i1> %Pg,
          i32 %prfop)

The intrinsics are the IR counterpart of the following SVE ACLE functions:

* void svprf<T>(svbool_t pg, const void *base, svprfop op)
* void svprf<T>_vnum(svbool_t pg, const void *base, int64_t vnum, svprfop op)
* void svprf<T>_gather[_u32base](svbool_t pg, svuint32_t bases, svprfop op)
* void svprf<T>_gather[_u64base](svbool_t pg, svuint64_t bases, svprfop op)
* void svprf<T>_gather_[s32]offset(svbool_t pg, const void *base, svint32_t offsets, svprfop op)
* void svprf<T>_gather_[u32]offset(svbool_t pg, const void *base, svint32_t offsets, svprfop op)
* void svprf<T>_gather_[s64]offset(svbool_t pg, const void *base, svint64_t offsets, svprfop op)
* void svprf<T>_gather_[u64]offset(svbool_t pg, const void *base, svint64_t offsets, svprfop op)
* void svprf<T>_gather[_u32base]_offset(svbool_t pg, svuint32_t bases, int64_t offset, svprfop op)
* void svprf<T>_gather[_u64base]_offset(svbool_t pg, svuint64_t bases,int64_t offset, svprfop op)

Reviewers: andwar, sdesmalen, efriedma, rengolin

Subscribers: tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75580