[GlobalISel][TableGen] Fix handling of default operands

[GlobalISel][TableGen] Fix handling of default operands

When looping through a destination pattern's operands to decide how many
default operands we need to introduce, we used to count the "expanded"
number of operands. So if one default operand would be rendered as 2
values, we'd count it as 2 operands, when in fact it needs to count as
only 1 operand regardless of how many values it expands to.

This turns out to be a problem only in some very specific cases, e.g.
when we have one operand with multiple default values followed by more
operands with default values (see the new test). In such a situation
we'd stop looping before looking at all the operands, and then error out
assuming that we don't have enough default operands to make up the
shortfall.

At the moment this only affects ARM.

The patch removes the loop counting default operands entirely and
assumes that we'll have to introduce values for any default operand that
we find (i.e. we're assuming it cannot be given as a child at all). It
also extracts the code for adding renderers for default operands into a
helper method.

Differential Revision: https://reviews.llvm.org/D33031

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303240 91177308-0d34-0410-b5e6-96231b3b80d8