[DebugInfo] Fix legacy ZExt emission when FromBits >= 64 (PR47927)

Fix an out-of-bounds shift in emitLegacyZExt by using a slightly more
complicated dwarf expression to create the zext mask.

This addresses a UBSan diagnostic seen when compiling compiler-rt
(llvm.org/PR47927).

rdar://70307714

Differential Revision: https://reviews.llvm.org/D89838