[WebAssembly] Use Function::hasOptSize() (NFC)

Summary: Use member function.

Reviewers: aheejin

Subscribers: sunfish, hiraditya, sbc100, jgravelle-google, dschuff, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60651

Patch by Hideto Ueno (uenoku)

llvm-svn: 358336