[SDAG] Don't duplicate ParseConstraints() implementation SDAGBuilder (NFCI)

visitInlineAsm() in SDAGBuilder was duplicating a lot of the code
in ParseConstraints(), in particular all the logic to determine the
operand value and constraint VT.

Rely on the data computed by ParseConstraints() instead, and update
its ConstraintVT implementation to match getCallOperandValEVT()
more precisely.