[NFC][compiler-rt] Cleanup Implicit Conversion Sanitizer tests to use sized types

Summary: As requested by @filcab in https://reviews.llvm.org/D50251#1280267

Reviewers: filcab, vsk, #sanitizers, vitalybuka

Reviewed By: filcab, #sanitizers, vitalybuka

Subscribers: vitalybuka, kubamracek, dberris, llvm-commits, filcab

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D53869

llvm-svn: 345661