__arithmetic_fence enforces ordering on expression evaluation when fast-math

is enabled.
In fast math mode some floating-point optimizations are performed such as
reassociation and distribution.

For example, the compiler may transform (a+b)+c into a+(b+c). Although these
two expressions are equivalent in integer arithmetic, they may not be in
floating-point arithmetic. The builtin tells the compiler that the expression
in parenthesis can’t be re-associated or distributed.
__arithmetic_fence(a+b)+c is not equivalent to a+(b+c).

This patch adds the support of the builtin to SPIR target.

Differential Revision: https://reviews.llvm.org/D142583