[WebAssembly] Update opcode name format for conversions

Summary:
Conversion opcode name format should be f64.convert_u/i64 not f64_convert_u

Author: s3ththompson
Reviewers: jfb
Subscribers: sunfish, jfb, llvm-commits, dschuff
Differential Revision: http://reviews.llvm.org/D14160

llvm-svn: 251613