[X86] In OptimizeLEAs pass, check that the key is in the LEAs map before accessing

When the key is not already in the map, the access operator[] creates an empty value and grows the map.
Resizing a map is very slow, so this needs to be avoided.

Found with csmith + asserts.

May help with
https://bugs.llvm.org/show_bug.cgi?id=25843

Patch by Tom Rix.

Differential Revision: https://reviews.llvm.org/D50780

llvm-svn: 340434