[ArgPromotion] Fix a truncated variable

This fixes a regression since SVN rev 273808 (which was supposed to
not change functionality).

The regression caused miscompilations (noted in the wild when targeting
AArch64) on platforms with 32 bit long.

Differential Revision: https://reviews.llvm.org/D32850

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302137 91177308-0d34-0410-b5e6-96231b3b80d8