[flang] Fix IsConstantExpr for division expressions

Summary:
Fortran::evaluate::IsConstantExpr did not check that the numerator
was a constant expression. This patch fixes the issue.

Reviewers: DavidTruby, klausler, schweitz, PeteSteinfeld, jdoerfert, sscalpone

Reviewed By: klausler, PeteSteinfeld, sscalpone

Subscribers: llvm-commits

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D81096