This patch aims to reduce compile time in LegalizeTypes by using SmallDenseMap,

with an initial number of elements,  instead of DenseMap, which has
zero initial elements, in order to avoid the copying of elements
when the size changes and to avoid allocating space every time
LegalizeTypes is run. This patch will not affect the memory footprint,
because DenseMap will increase the element size to 64
when the first element is added.

Patch by Wan Xiaofei.

llvm-svn: 173448