Fix typo that snuck into one of the vasm diffs.

Summary: Looking at the way newIdx is used, the typo means we could get
down the "idx not const" path, then use newIdxReg which should
be valid, because ContArUpdateIdx is not whitelisted in
mayUseConst(). I think we're dodging the bullet but this needs
fixing anyway.

Reviewed By: @swtaarrs

Differential Revision: D1536301