[Constants] Simplify type checking switch in ConstantExpr::get.

Summary:
Remove duplicate checks that both operands have the same type. This is checked
before the switch.

Use 'integer' or 'floating-point' instead of 'arithmetic' type. I think this
might be a leftover to the days when floating point and integer operations
shared the same opcodes.

Reviewers: spatel, RKSimon, dblaikie

Reviewed By: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61558

llvm-svn: 359985