[flang] Use X->foo() instead of X.getValue().foo() (NFC)

Flang C++ Style Guide tells us to use *X when the reference is
protected by a presense test.  However, (*X).foo() is a little harder
to read, especially when X is a complicated expression.

This patch slightly deviates from the guide (but retains the spirit)
by using X->foo() instead.

Differential Revision: https://reviews.llvm.org/D130413