[BOLT][NFC] Fix UB due to left shift of negative value

The following test fails when enabling UBSan due to a left shift of a
negative value:

> runtime error: left shift of negative value -2

  BOLT :: AArch64/ext-island-ref.s

This patch fixes this by using a multiplication instead of a shift.

Reviewed By: yota9

Differential Revision: https://reviews.llvm.org/D148218