[AsmWriter] Eliminate warning. NFC

This patch eliminates the following warning

lib/IR/AsmWriter.cpp:1128:57: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
                 (StrVal[1] >= '0' && StrVal[1] <= '9')) &&

Reviewers: timshen, rnk, davide
Reviewed By: davide
Subscribers: davide, llvm-commits
Differential Revision: https://reviews.llvm.org/D32337

llvm-svn: 300950