[llvm-mca] Fix PR38575: Avoid an invalid implicit truncation of a processor...

[llvm-mca] Fix PR38575: Avoid an invalid implicit truncation of a processor resource mask (an uint64_t value) to unsigned.

This patch fixes a regression introduced at revision 338702.

A processor resource mask was incorrectly implicitly truncated to an unsigned
quantity. Later on, the truncated mask was used to initialize an element of a
vector of processor resource descriptors.
On targets with more than 32 processor resources, some elements of the vector
are left uninitialized. As a consequence, this bug might have eventually caused
a crash due to null dereference in the Scheduler.

This patch fixes PR38575, and adds a test for it.

llvm-svn: 339768