WebAssembly: print immediates

Summary:
For now output using C99's hexadecimal floating-point representation.

This patch also cleans up how machine operands are printed: instead of special-casing per type of machine instruction, the code now handles operands generically.

Reviewers: sunfish

Subscribers: llvm-commits, jfb

Differential Revision: http://reviews.llvm.org/D11914

llvm-svn: 244520