[ARM] Fix an off-by-one error when restoring LR for 16-bit Thumb

The commit https://reviews.llvm.org/rL318143 computes incorrectly to offset to
restore LR from.

The number of tPOP operands is 2 (condition) + 2 (implicit def and use of SP) +
count of the popped registers. We need to load LR from just past the last
register, hence the correct offset should be either getNumOperands() - 4 and
getNumExplicitOperands() - 2 (multiplied by 4).

Differential revision: https://reviews.llvm.org/D40305

llvm-svn: 319014