[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().
git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@308630 91177308-0d34-0410-b5e6-96231b3b80d8