[FileCheck] Stop qualifying expressions as numeric

Summary:
Stop referring to "numeric expression", using simply the term
"expression" instead. Likewise for numeric operation since operations
are only used in numeric expressions.

Reviewers: jhenderson, jdenny, probinson, arichardson

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63500

llvm-svn: 363901