[X86] Use a std::vector for the memory unfolding table.

Previously we used a DenseMap which is costly to set up due to multiple full table rehashes as the size increases and causes the table to be reallocated.

This patch changes the table to a vector of structs. We now walk the reg->mem tables and push new entries in the mem->reg table for each row not marked TB_NO_REVERSE. Once all the table entries have been created, we sort the vector. Then we can use a binary search for lookups.

Differential Revision: https://reviews.llvm.org/D48585

llvm-svn: 335994