[ScopInfo] Use map for lookupPHIReadOf. NFC.

Introduce previously missing PHIReads analogous the the already existing
PHIWrites/ValueWrites/ValueReads maps. PHIReads was initially not
required and the later introduced lookupPHIReadOf() used a linear
search instead.

With PHIReads, lookupPHIReadOf() can now also do a map lookup and remove
any surprising performance/behaviour differences to lookupPHIWriteOf(),
lookupValueWriteOf() and lookupValueReadOf().

llvm-svn: 308630