[test] Improve FileCheck's numeric-expression.txt

Summary:
Various improvement for FileCheck's numeric-expression.txt test:
- remove unused values in USE DEF FMT IMPL MATCH section
- replace 14 literal for 0xe and 0xE to have example of hex literals
- rename variable to be more self-descriptive
- move CHECK as comment of the values being matched to help readability
- add conversion tests
- simplify test for use of several numeric variables by using existing
  variable
- adjust position of error message check to match the alignment of the
  error message wrt. the output matched by the previous check

Reviewed By: jhenderson, jdenny

Differential Revision: https://reviews.llvm.org/D79820