[NVPTX] Mark special registers as reserved

A reserved register:
 - is not allocatable
 - is considered always live
 - is ignored by liveness tracking

NVPTX special registers match the criteria, and marking them as
reserved helps to avoid machine verifier error:

    *** Bad machine code: Using an undefined physical register ***
    - function:    foo
    - basic block: %bb.0  (0x557bb178b708)
    - instruction: %0:int32regs = MOV_SPECIAL $envreg0
    - operand 1:   $envreg0

Differential Revision: https://reviews.llvm.org/D113008