Fix UB in DwarfExpression::emitLegacyZExt()

A shift-left > 63 triggers a UBSAN failure. This patch kicks the can
down the road (to the consumer) by emitting a more compact
representation of the shift computation in DWARF expressions.

Relanding (I accidentally pushed an earlier version of the patch previously).

Differential Revision: https://reviews.llvm.org/D118183