[MC] Relax .fill size requirements

Avoid requirement that number of values must be known at assembler
time.

Fixes PR33586.

Reviewers: rnk, peter.smith, echristo, jyknight

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D46703

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332741 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed