Implement matching constraints. We can now say things like this:

Implement matching constraints.  We can now say things like this:

%C = call int asm "xyz $0, $1, $2, $3", "=r,r,r,0"(int %A, int %B, int 4)

and get:

xyz r2, r3, r4, r2

note that the r2's are pinned together.  Yaay for 2-address instructions.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25893 91177308-0d34-0410-b5e6-96231b3b80d8