[NFC][AsmPrinter] Windows warning: Use explicit cast

static_cast for uint64_t to unsigned gives a MS VC build warning
for Windows:

warning C4309: 'static_cast': truncation of constant value

Use an explicit cast instead.

Change-Id: I692d335b4913070686a102780c1fb05b893a2f69

Differential Revision: https://reviews.llvm.org/D94592