[GVN] Remove stale entries in phitranslate cache when new phi is generated for PRE

When a new phi is generated for scalarpre of an expression, the phiTranslate cache
will become stale: Before PRE, the candidate expression must not be available in a
predecessor block, and phitranslate will cache the information. After PRE, the
expression will become available in all predecessor blocks, so the related entries
in phiTranslate cache becomes stale. The patch will simply remove the stale entries
so phiTranslate can be recomputed next time.

The stale entries in phitranslate cache will not affect correctness but will cause
missing PRE opportunity for later instructions.

Differential Revision: https://reviews.llvm.org/D36124

llvm-svn: 310421