[NVPTX] Fix pointer argument declaration for --nvptx-short-ptr

When --nvptx-short-ptr is set, local pointers are stored as 32-bit on
nvptx64 target.

Before this patch, arguments for a function declaration were always
emitted as b64 regardless of their address space, but they were set as
b32 for the corresponding call instruction:

   .extern .func test
   (
    .param .b64 test_param_0
   )
   [...]
    .param .b32 param0;
    st.param.b32 [param0+0], %r1;
    call.uni test, (param0);

This is not supported:

  ptxas: Type of argument does not match formal parameter
  'test_param_0'

Now short pointers in a function declaration are emitted as b32 if
--nvptx-short-ptr is set.

Differential Revision: https://reviews.llvm.org/D135674