[GlobalISel] Replace all combined G_EXTRACT uses.

[GlobalISel] Replace all combined G_EXTRACT uses.

Iterating on the use-list we're modifying doesn't work: after the first
iteration, the use-list iterator will point to a MachineOperand
referencing the new register.  This caused us to skip the other uses to
replace.

Instead, use MRI.replaceRegWith(), which accounts for this behavior.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296551 91177308-0d34-0410-b5e6-96231b3b80d8