[Sparc] Check register use with isPhysRegUsed() instead of reg_nodbg_empty()

Summary: By using reg_nodbg_empty() to determine if a function can be
treated as a leaf function or not, we miss the case when the register
pair L0_L1 is used but not L0 by itself. This has the effect that
use_all_i32_regs(), a test in reserved-regs.ll which tries to use all
registers, gets treated as a leaf function.

Reviewers: jyknight, venkatra

Reviewed By: jyknight

Subscribers: davide, RKSimon, sepavloff, llvm-commits

Differential Revision: https://reviews.llvm.org/D27089

llvm-svn: 297285