[WebAssembly] tablegen: distinguish float/int immediate operands.

Summary:
Before, they were one category of operands which could cause
crashes in non-sensical combinations, e.g. "f32.const symbol".
Now these are forced to be an error.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, aheejin, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64039

llvm-svn: 365351